attr: fix instructions on how to check attrs

The instructions in attr.h describing what functions to call to check
attributes is missing the index as the first argument to
git_check_attr(), as well as tree_oid as the second argument.

When 7a400a2c (attr: remove an implicit dependency on the_index,
2018-08-13) started passing an index_state instance to git_check_attr(),
it forgot to update the API documentation in
Documentation/technical/api-gitattributes.txt. Later, 3a1b3415
(attr: move doc to attr.h, 2019-11-17) moved the API documentation to
attr.h as a comment, but still left out the index_state as an argument.

In 47cfc9b (attr: add flag `--source` to work with tree-ish 2023-01-14)
added tree_oid as an optional parameter but was not added to the docs in
attr.h

Fix this to make the documentation in the comment consistent with the
actual function signature.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

attr.h

@@ -45,7 +45,7 @@
  * const char *path;
  *
  * setup_check();
- * git_check_attr(path, check);
+ * git_check_attr(&the_index, tree_oid, path, check);
  * ------------
  *
  * - Act on `.value` member of the result, left in `check->items[]`: