shell doc: remove stray "+" in example

The git-shell(1) manpage says

	EXAMPLE
	       To disable interactive logins, displaying a greeting
		instead:

		+

		   $ chsh -s /usr/bin/git-shell
		   $ mkdir $HOME/git-shell-commands
[...]

The stray "+" has been there ever since the example was added in
v1.8.3-rc0~210^2 (shell: new no-interactive-login command to print a
custom message, 2013-03-09).  The "+" sign between paragraphs is
needed in asciidoc to attach extra paragraphs to a list item but here
it is not needed and ends up rendered as a literal "+".  Remove it.

A quick search with "grep -e '<p>+' /usr/share/doc/git/html/*.html"
doesn't find any other instances of this problem.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-shell.txt

@@ -66,7 +66,7 @@ EXAMPLE
 -------
 
 To disable interactive logins, displaying a greeting instead:
-+
+
 ----------------
 $ chsh -s /usr/bin/git-shell
 $ mkdir $HOME/git-shell-commands