Add GH issue number to TODO comment

Author: Pyry Kovanen

pandas/tests/io/json/test_json_table_schema.py

@@ -567,6 +567,6 @@ def test_empty_frame_roundtrip(self):
         expected = df.copy()
         out = df.to_json(orient='table')
         result = pd.read_json(out, orient='table')
-        # TODO: When DF coercion issue (GH ) is resolved, tighten type checks
+        # TODO: After DF coercion issue (GH 21345) is resolved, tighten type checks
         tm.assert_frame_equal(expected, result,
                               check_dtype=False, check_index_type=False)