Fix indent and rendering across 4 different examples (#741)

This patch fixes indent and rendering across 4 different examples.

Author: jungseoklee

src/database/postgres/aggregate_data.md

@@ -27,10 +27,10 @@ fn main() -> Result<(), Error> {
         
         if nationality.is_some () && count.is_some () {
 
-            let nation = Nation{
+            let nation = Nation {
                 nationality: nationality.unwrap(),
                 count: count.unwrap(),
-        };
+            };
             println!("{} {}", nation.nationality, nation.count);
             
         }

src/development_tools/versioning/semver-prerelease.md

@@ -17,5 +17,3 @@ fn main() -> Result<(), Error> {
     Ok(())
 }
 ```
-
-[`

src/encoding/csv/serde-serialize.md

@@ -32,3 +32,4 @@ fn main() -> Result<()> {
 
     Ok(())
 }
+```

src/file/dir/duplicate-name.md

@@ -26,3 +26,4 @@ fn main() {
         }
     }
 }
+```