Fix markdown inline code typo

Author: felipedmesquita

guides/source/active_support_core_extensions.md

@@ -4088,7 +4088,7 @@ Extensions to Pathname
 
 ### `existence`
 
-The [`existence`][Pathname#existence] method returns the receiver if the named file exists otherwise returns +nil+. It is useful for idioms like this:
+The [`existence`][Pathname#existence] method returns the receiver if the named file exists otherwise returns `nil`. It is useful for idioms like this:
 
 ```ruby
 content = Pathname.new("file").existence&.read