mkUserGuide: fix option wrapping in a table

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>

utils/mkUserGuidePart/Main.hs

@@ -48,7 +48,7 @@ whatGlasgowExtsDoes = unlines
 -- the users guide.
 flagsTable :: [Flag] -> ReST
 flagsTable theFlags =
-    table [50, 100, 30, 50]
+    table [50, 100, 30, 55]
           ["Flag", "Description", "Static/Dynamic", "Reverse"]
           (map flagRow theFlags)
   where