Update usage section example commands

Change <repo> to <local-repo> so that it's clear that we invoke from a local repository;
Add 'git checkout HEAD' command as we need to run it as the final step.

Thanks

Author: ceqi

README.md

@@ -33,7 +33,8 @@ Using hg-fast-export is quite simple for a mercurial repository <repo>:
 mkdir repo-git # or whatever
 cd repo-git
 git init
-hg-fast-export.sh -r <repo>
+hg-fast-export.sh -r <local-repo>
+git checkout HEAD
 ```
 
 Please note that hg-fast-export does not automatically check out the