DOC: acl is http_first_req, not http_req_first

Doc was entered without correctly copying the ACL name.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 9486b95..c6785c2 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -7747,7 +7747,7 @@
 
   Currently only http basic auth is supported.
 
-http_req_first
+http_first_req
   Returns true when the request being processed is the first one of the
   connection. This can be used to add or remove headers that may be missing
   from some requests when a request is not the first one, or even to perform