git-svn documentation: minor grammar fix

Use the definite article when talking about a configuration property.
Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: NEric Wong <normalperson@yhbt.net>

Documentation/git-svn.txt

@@ -243,7 +243,7 @@ where <name> is the name of the SVN repository as specified by the -R option to
 
 --username;;
 	Specify the SVN username to perform the commit as.  This option overrides
-	configuration property 'username'.
+	the 'username' configuration property.
 
 --commit-url;;
 	Use the specified URL to connect to the destination Subversion