index.h: Correct documentation for git_index_open_inrepo()

Fix the doxygen comments for git_index_open_inrepo(). Previously they
referred to a param index_path and omitted index (probably a c&p
error).

include/git2/index.h

@@ -91,8 +91,8 @@ GIT_EXTERN(int) git_index_open_bare(git_index **index, const char *index_path);
  * Open the Index inside the git repository pointed
  * by 'repo'.
  *
+ * @param index the pointer for the new index
  * @param repo the git repo which owns the index
- * @param index_path the path to the index file in disk
  * @return 0 on success; error code otherwise
  */
 GIT_EXTERN(int) git_index_open_inrepo(git_index **index, git_repository *repo);