Correct grammar in '... attempt to escape whitespace' diagnostic.

Author: trwyant

pod/perldiag.pod

@@ -5501,7 +5501,7 @@ You probably wrote something like this:
         another
     );
 
-expecting to get a two elements list containing the strings C<'a string'>
+expecting to get a two element list containing the strings C<'a string'>
 and C<'another'>.  Instead the list will hold three elements: C<'a\'>
 (with a literal backslash), C<'string'> and C<'another'>.