Noticed as a sphinx warning:
    docs/users_guide/flags-warnings.gen.rst:97:
        WARNING: Inline interpreted text or phrase
        reference start-string without end-string.

Which pointed to broken table.

Before the patch table looked like:

    | :ghc-flag:`-Wno-unticked-promoted-constructors |
    | `                                              |

After the patch long link is on a single line:

    | :ghc-flag:`-Wno-unticked-promoted-constructors`    |

Signed-off-by: Sergei Trofimovich <siarheit@google.com>