doc: fix mistake about matching against directories in "pattern.txt"

This fixes mistake of documentation about matching against directories
in "pattern.txt" introduced by 50db996bccaf.

".hgignore" treats specified "glob:" pattern as same as one specified
for "-X" option: it can match against directories, too.

For reference, extra regexp string appended to specified pattern for
each types are listed below: see also "match.match()" and
"match._regex()" for detail.

  ============= ========== ===============
  type          cmdline    -I/-X
  ============= ========== ===============
  glob/relglob  '$'        '(?:/|$)'
  path/relpath  '(?:/|$)'  '(?:/|$)'
  re/relre      (none)     (none)
  ============= ========== ===============

Appending '$' means that the specified pattern should match against
only files.

mercurial/help/patterns.txt

@@ -30,9 +30,8 b' feeds. Each string read from the file is'
 pattern.
 
 All patterns, except for ``glob:`` specified in command line (not for
-``-I`` or ``-X`` options) or ``.hgignore`` file, can match also
-against directories: files under matched directories are treated as
-matched.
+``-I`` or ``-X`` options), can match also against directories: files
+under matched directories are treated as matched.
 
 Plain examples::