Fix error message for unknown global parameter

When an unknown global parameter was modified, the error message
mistakenly logged onl the value of the parameter instead of the name and
value.

server/core/config_runtime.cc

@@ -705,7 +705,7 @@ bool runtime_alter_maxscale(const char* name, const char* value)
     }
     else
     {
-        runtime_error("Unknown global parameter: %s", value);
+        runtime_error("Unknown global parameter: %s=%s", name, value);
     }
 
     if (rval)