[DOC] fix unescaped space in httpchk example.

Lars Braeuer reported a missing space in the example which drives
readers in wrong direction.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index e79a437..7093c78 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -1869,7 +1869,7 @@
       # using HTTP request "OPTIONS * HTTP/1.1" on port 80.
       backend https_relay
           mode tcp
-          option httpchk OPTIONS * HTTP/1.1\r\nHost: www
+          option httpchk OPTIONS * HTTP/1.1\r\nHost:\ www
           server apache1 192.168.1.1:443 check port 80
 
   See also : "option ssl-hello-chk", "option smtpchk", "http-check" and the