Fix help string to avoid PHP notice

Escape a $ symbol in help message and rephrase slightly.  Without this,
`php maintenance/language/checkDupeMessages.php --help` prints
	PHP Notice:  Undefined variable: dupeMessages in
        language/checkDupeMessages.php on line 56

Change-Id: Ie7c0f07e68cee1f7cd2758f71b09d34e339d4b7b