Add "Remarks" and "Example" headings in C2018 error reference

Author: Rageking8

docs/error-messages/compiler-errors-1/compiler-error-c2018.md

@@ -10,8 +10,12 @@ ms.assetid: 86b54573-dca0-4446-be1a-e3ac489c073b
 
 > unknown character 'hexnumber'
 
+## Remarks
+
 The source file contains an unexpected ASCII character, which is identified by its hex number. To resolve the error, remove the character.
 
+## Example
+
 The following sample generates C2018:
 
 ```cpp