[4.0] 00c133a Fix example

[Lasse Karstensen]

commit 00c133a9b3be1a5b3a0efc486d08b45e58c1e515
Author: Federico G. Schwindt <fgsch at lodoss.net>
Date:   Wed Nov 12 17:01:06 2014 +0000

    Fix example
    
    backslash is used for escaping in varnish-cli, so escape the backslash
    to actually pass "\.png$".

diff --git a/doc/sphinx/users-guide/purging.rst b/doc/sphinx/users-guide/purging.rst
index 70fb415..43df304 100644
--- a/doc/sphinx/users-guide/purging.rst
+++ b/doc/sphinx/users-guide/purging.rst
@@ -73,7 +73,7 @@ Support for bans is built into Varnish and available in the CLI
 interface. To ban every png object belonging on example.com, issue
 the following command::
 
-  ban req.http.host == "example.com" && req.url ~ "\.png$"
+  ban req.http.host == "example.com" && req.url ~ "\\.png$"
 
 Quite powerful, really.