[clang-format][doc] Add the link to POSIX 2.13

Author: owenca

clang/docs/ClangFormat.rst

@@ -142,8 +142,9 @@ names. It has the following format:
 * The slash (``/``) is used as the directory separator.
 * A pattern is relative to the directory of the ``.clang-format-ignore`` file
   (or the root directory if the pattern starts with a slash).
-* Patterns follow the rules specified in POSIX 2.13.1, 2.13.2, and Rule 1 of
-  2.13.3.
+* Patterns follow the rules specified in `POSIX 2.13.1, 2.13.2, and Rule 1 of
+  2.13.3 <https://pubs.opengroup.org/onlinepubs/9699919799/utilities/
+  V3_chap02.html#tag_18_13>`_.
 * A pattern is negated if it starts with a bang (``!``).
 
 To match all files in a directory, use e.g. ``foo/bar/*``. To match all files in