Documentation: mention module option to git-cvsimport

The git-cvsimport argument that specifies a cvs module to import should
probably be included in the default example.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/cvs-migration.txt

@@ -109,7 +109,7 @@ sure it is in your path.  Then cd to a checked out CVS working directory
 of the project you are interested in and run gitlink:git-cvsimport[1]:
 
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-$ git cvsimport -C <destination>
+$ git cvsimport -C <destination> <module>
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 This puts a git archive of the named CVS module in the directory