Correct regexp in examples

Fixes #2380.

doc/sphinx/reference/varnish-cli.rst

@@ -426,7 +426,7 @@ Example: Ban all documents where the serving host is "example.com"
 or "www.example.com", and where the Set-Cookie header received from
 the backend contains "USERID=1663"::
 
-    req.http.host ~ "^(?i)(www\.)example.com$" && obj.http.set-cookie ~ "USERID=1663"
+    ban req.http.host ~ "^(?i)(www\\.)?example\\.com$" && obj.http.set-cookie ~ "USERID=1663"
 
 AUTHORS
 =======

doc/sphinx/reference/vcl.rst

@@ -145,7 +145,7 @@ To send flags to the PCRE engine, such as to do case insensitive matching, add
 the flag within parens following a question mark, like this::
 
     # If host is NOT example dot com..
-    if (req.http.host !~ "(?i)example.com$") {
+    if (req.http.host !~ "(?i)example\.com$") {
         ...
     }