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.

(cherry picked from commit d49b559a15a8781ba37877812d4d3ddf2cea9eb4)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 6e7ac7cf7dff1c625dbd62929e075598c8684a1a)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit eb7eea02705a1134e9c2221a7fccfa5a5af08f55)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 5bab248..0a560d4 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -12424,7 +12424,7 @@
   evaluated.
 
   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: