[Tree Building Exercise & Class Inheritance Concept] Spelling Fixes (#4009)

* Fix possessive 'its' spelling in documentation and tests
* Corrected 'possibly' to 'possible
* Revert "Corrected 'possibly' to 'possible"
This reverts commit 9a42041.
* revert: changes in reference/ folder

[no important files changed]

Author: MarkRosemaker

concepts/class-inheritance/about.md

@@ -7,7 +7,7 @@ In situations where only a small amount of functionality needs to be customized
 
 `Inheritance` describes `is a kind of` relationship between two or more classes, abstracting common details into super (_base_ or _parent_) class and storing specific ones in the subclass (_derived class_ or _child class_).
 
-To create a child class, specify the parent class name inside the pair of parenthesis, followed by it's name.
+To create a child class, specify the parent class name inside the pair of parenthesis, followed by its name.
 Example
 ```python
 class Child(Parent):  

exercises/practice/tree-building/.meta/example.py

@@ -18,7 +18,7 @@ def validate_record(record):
         raise ValueError('Only root should have equal record and parent id.')
 
     if not record.equal_id() and record.parent_id >= record.record_id:
-        raise ValueError("Node parent_id should be smaller than it's record_id.")
+        raise ValueError("Node parent_id should be smaller than its record_id.")
 
 
 def BuildTree(records):

exercises/practice/tree-building/tree_building_test.py

@@ -111,7 +111,7 @@ def test_root_node_has_parent(self):
         with self.assertRaises(ValueError) as err:
             BuildTree(records)
         self.assertEqual(type(err.exception), ValueError)
-        self.assertEqual(err.exception.args[0], "Node parent_id should be smaller than it's record_id.")
+        self.assertEqual(err.exception.args[0], "Node parent_id should be smaller than its record_id.")
 
     def test_no_root_node(self):
         records = [
@@ -167,7 +167,7 @@ def test_cycle_indirectly(self):
         with self.assertRaises(ValueError) as err:
             BuildTree(records)
         self.assertEqual(type(err.exception), ValueError)
-        self.assertEqual(err.exception.args[0], "Node parent_id should be smaller than it's record_id.")
+        self.assertEqual(err.exception.args[0], "Node parent_id should be smaller than its record_id.")
 
     def test_higher_id_parent_of_lower_id(self):
         records = [
@@ -179,7 +179,7 @@ def test_higher_id_parent_of_lower_id(self):
         with self.assertRaises(ValueError) as err:
             BuildTree(records)
         self.assertEqual(type(err.exception), ValueError)
-        self.assertEqual(err.exception.args[0], "Node parent_id should be smaller than it's record_id.")
+        self.assertEqual(err.exception.args[0], "Node parent_id should be smaller than its record_id.")
 
     def assert_node_is_branch(self, node, node_id, children_count):
         self.assertEqual(node.node_id, node_id)