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>