DOC: Fix typo in iif() example

It should read 'iif', not 'iff'.

Reported-by: Miroslav Zagorac <zaga@fly-etf.net>
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 0e34f62..075d3d5 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -15155,7 +15155,7 @@
   string otherwise.
 
   Example:
-    http-request set-header x-forwarded-proto %[ssl_fc,iff(https,http)]
+    http-request set-header x-forwarded-proto %[ssl_fc,iif(https,http)]
 
 in_table(<table>)
   Uses the string representation of the input sample to perform a look up in