book: Change "Failures" to "Errors" in doc special sections chapter

This matches the usage in the standard library's documentation.

Author: kamalmarhubi

src/doc/book/documentation.md

@@ -118,7 +118,7 @@ least. If your function has a non-trivial contract like this, that is
 detected/enforced by panics, documenting it is very important.
 
 ```rust
-/// # Failures
+/// # Errors
 # fn foo() {}
 ```