gitmodules(5): remove leading period from synopsis

Asciidoc treats a line starting with a period followed by a title as a
blocktitle element. My introduction of gitmodules(5) unfortunatly broke
the documentation build process due to this processing, since it made
asciidoc generate an illegal (empty) synopsis element. Removing the leading
period fixes the problem and also makes gitmodules(5) use the same synopsis
notation as gitattributes(5).
Noticed-by: NMatthias Lederhofer <matled@gmx.net>
Signed-off-by: NLars Hjemli <hjemli@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/gitmodules.txt

@@ -7,7 +7,7 @@ gitmodules - defining submodule properties
 
 SYNOPSIS
 --------
-.gitmodules
+gitmodules
 
 
 DESCRIPTION