gitignore.txt: fix documentation of "**" patterns

"**" means bold in ASCIIDOC, so we need to escape it.
Signed-off-by: NKarsten Blees <blees@dcon.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/gitignore.txt

@@ -113,12 +113,12 @@ full pathname may have special meaning:
 
  - A leading "`**`" followed by a slash means match in all
    directories. For example, "`**/foo`" matches file or directory
-   "`foo`" anywhere, the same as pattern "`foo`". "**/foo/bar"
+   "`foo`" anywhere, the same as pattern "`foo`". "`**/foo/bar`"
    matches file or directory "`bar`" anywhere that is directly
    under directory "`foo`".
 
- - A trailing "/**" matches everything inside. For example,
-   "abc/**" matches all files inside directory "abc", relative
+ - A trailing "`/**`" matches everything inside. For example,
+   "`abc/**`" matches all files inside directory "`abc`", relative
    to the location of the `.gitignore` file, with infinite depth.
 
  - A slash followed by two consecutive asterisks then a slash