gitfaq: avoid validation error with older asciidoc

When building with asciidoc-8.4.5 (as found on CentOS/Red Hat 6), the
period in the "[[files-in-.gitignore-are-tracked]]" anchor is not
properly parsed as a section:

  WARNING: gitfaq.txt: line 245: missing [[files-in-.gitignore-are-tracked]] section

The resulting XML file fails to validate with xmlto:

    xmlto: /git/Documentation/gitfaq.xml does not validate (status 3)
    xmlto: Fix document syntax or use --skip-validation option
     /git/Documentation/gitfaq.xml:3: element refentry: validity error :
     Element refentry content does not follow the DTD, expecting
     (beginpage? , indexterm* , refentryinfo? , refmeta? , (remark | link
     | olink | ulink)* , refnamediv+ , refsynopsisdiv? , (refsect1+ |
     refsection+)), got (refmeta refnamediv refsynopsisdiv refsect1
     refsect1 refsect1 refsect1 variablelist refsect1 refsect1 )
    Document /git/Documentation/gitfaq.xml does not validate

Let's avoid breaking users of platforms which ship an old version of
asciidoc, since the cost to do so is quite low.

Reported-by: Son Luong Ngoc <[email protected]>
Signed-off-by: Todd Zullinger <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: tmzullinger

Documentation/gitfaq.txt

@@ -223,7 +223,7 @@ a file checked into the repository which is a template or set of defaults which
 can then be copied alongside and modified as appropriate.  This second, modified
 file is usually ignored to prevent accidentally committing it.
 
-[[files-in-.gitignore-are-tracked]]
+[[files-in-gitignore-are-tracked]]
 I asked Git to ignore various files, yet they are still tracked::
 	A `gitignore` file ensures that certain file(s) which are not
 	tracked by Git remain untracked.  However, sometimes particular