git-bugreport.txt: adjust reference to strftime(3)

The strftime(3) man page is outside of the Git suite.  Refererence it as
we do other external man pages and avoid creating a broken link when
generating the HTML documentation.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: tmzullinger

Documentation/git-bugreport.txt

@@ -45,7 +45,7 @@ OPTIONS
 --suffix <format>::
 	Specify an alternate suffix for the bugreport name, to create a file
 	named 'git-bugreport-<formatted suffix>'. This should take the form of a
-	link:strftime[3] format string; the current local time will be used.
+	strftime(3) format string; the current local time will be used.
 
 GIT
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