Documentation/interpret-trailers: Grammar fix

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: tklauser

Documentation/git-interpret-trailers.txt

@@ -67,7 +67,7 @@ OPTIONS
 --trim-empty::
 	If the <value> part of any trailer contains only whitespace,
 	the whole trailer will be removed from the resulting message.
-	This apply to existing trailers as well as new trailers.
+	This applies to existing trailers as well as new trailers.
 
 --trailer <token>[(=|:)<value>]::
 	Specify a (<token>, <value>) pair that should be applied as a