change the error call

Author: wistefan

apisix/plugins/opa/helper.lua

@@ -37,7 +37,7 @@ end
 local function get_body_for_request()
     local original_body, err = core.request.get_body()
     if err ~= nil then
-        error("opa - failed to get request body: ", err)
+        error("opa - failed to get request body: " .. err)
     end
     if original_body == nil then
         return nil