Fix list of released versions in the toc document

Author: gitster

Documentation/git.txt

@@ -43,7 +43,7 @@ unreleased) version of git, that is available from 'master'
 branch of the `git.git` repository.
 Documentation for older releases are available here:
 
-* link:v1.6.5/git.html[documentation for release 1.6.5]
+* link:v1.6.5.1/git.html[documentation for release 1.6.5.1]
 
 * release notes for
   link:RelNotes-1.6.5.1.txt[1.6.5.1],