glossary-content.txt: fix documentation of "**" patterns

"**" means bold in ASCIIDOC, so we need to escape it. This is similar
to 8447dc89 (gitignore.txt: fix documentation of "**" patterns -
2013-11-07)
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/glossary-content.txt

@@ -362,12 +362,12 @@ 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