Fix git(1) link to git-index-pack

Signed-off-by: NJonas Fonseca <fonseca@diku.dk>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git.txt

@@ -67,7 +67,7 @@ gitlink:git-commit-tree[1]::
 gitlink:git-hash-object[1]::
 	Computes the object ID from a file.
 
-gitlink:git-index-pack.html[1]::
+gitlink:git-index-pack[1]::
 	Build pack index file for an existing packed archive.
 
 gitlink:git-init-db[1]::