Fix plural ELEMENT and ALLOW_ATTR in docs

These were left over from their initial names. Make them singular to
conform with the current expectations.

Signed-off-by: Alexander Scheel <[email protected]>

Author: cipherboy

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -594,7 +594,7 @@ REGEXP = ^\s*((math(\s+|$)|inline(\s+|$)|display(\s+|$)))+
  - `ALLOW_ATTR`: The attribute this policy allows. Must be non-empty.
  - `REGEXP`: A regex to match the contents of the attribute against. Must be present but may be empty for unconditional whitelisting of this attribute.
 
-You may redefine `ELEMENTS`, `ALLOW_ATTRS`, and `REGEXP` multiple times; each time all three are defined is a single policy entry.
+You may redefine `ELEMENT`, `ALLOW_ATTR`, and `REGEXP` multiple times; each time all three are defined is a single policy entry.
 
 ## Time (`time`)