update check_http

docs/20_Checks/check_http.md

@@ -47,6 +47,7 @@ PARAMETERS:
   Define request:
   -u URL           Set url to fetch; eg. https://www.example.com/
   -m METHOD        Set a method, eg. HEAD; default: GET
+  -c PARAMS        additional curl params
 
   What to check:
   -s STATUSCODE    exact Statuscode to check; 3 digits; by default critical