Use singular

Author: Planeshifter

lib/node_modules/@stdlib/regexp/whitespace/README.md

@@ -87,7 +87,7 @@ bool = reWhitespace.REGEXP.test( '\\n' );
     /([\u0009\u000A\u000B\u000C\u000D\u0020\u0085\u00A0\u1680\u2000\u2001\u2002\u2003\u2004\u2005\u2006\u2007\u2008\u2009\u200A\u2028\u2029\u202F\u205F\u3000\uFEFF])/
     ```
 
--   The following characters can be set as flags in a regular expressions:
+-   The following characters can be set as flags in a regular expression:
 
     | Flag | Description         |
     | ---- | ------------------- |