DOC: configuration: missing 'if' in tcp-request content example

An example given for tcp-request content rule with lua
was missing 'if' keyword. Using it "as is" makes haproxy unhappy.

The example was introduced with 579d83b05.
So it may be backported as far as 1.6, but it is a really minor typo.

doc/configuration.txt

@@ -12748,7 +12748,7 @@ tcp-request content <action> [{if | unless} <condition>]
   available.
 
   Example:
-    tcp-request content use-service lua.deny { src -f /etc/haproxy/blacklist.lst }
+    tcp-request content use-service lua.deny if { src -f /etc/haproxy/blacklist.lst }
 
   Example: