docs: fix filter-branch example for quoted paths

If there is a quoted path, update-index will correctly
unquote it. However, we must take care to put our new prefix
inside the double-quote.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -358,7 +358,7 @@ To move the whole tree into a subdirectory, or remove it from there:
 
 ---------------------------------------------------------------
 git filter-branch --index-filter \
-	'git ls-files -s | sed "s-\t-&newsubdir/-" |
+	'git ls-files -s | sed "s-\t\"*-&newsubdir/-" |
 		GIT_INDEX_FILE=$GIT_INDEX_FILE.new \
 			git update-index --index-info &&
 	 mv $GIT_INDEX_FILE.new $GIT_INDEX_FILE' HEAD