Use Markdown's code spans instead of Emacs style `' (#186)

README.md

@@ -182,7 +182,7 @@ Possible known values are:
 Buffer local minor-mode to use to trim trailing whitespaces.
 
 If set, enable that mode when `trim_trailing_whitespace` is set to true.
-Otherwise, use `delete-trailing-whitespace'.
+Otherwise, use `delete-trailing-whitespace`.
 
 One possible value is
 [`ws-butler-mode`](https://github.com/lewang/ws-butler), with which