user-manual: fix anchor name Finding-comments-With-given-Content

Change the anchor name to

Finding-commits-With-given-Content

so that it corresponds to the actual content there.
Signed-off-by: NMichael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -955,7 +955,7 @@ echo "git diff --stat --summary -M v$last v$new > ../diffstat-$new"
 and then he just cut-and-pastes the output commands after verifying that
 they look OK.
 
-[[Finding-comments-With-given-Content]]
+[[Finding-commits-With-given-Content]]
 Finding commits referencing a file with given content
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~