doc: fix doc example for cctest

cctest guide example wasn't working because of a few typos.

Author: Matheus Marchini

doc/guides/writing-tests.md

@@ -312,8 +312,8 @@ TEST_F(EnvTest, RunAtExit) {
   v8::Local<v8::Context> context = v8::Context::New(isolate_);
   node::IsolateData* isolateData = node::CreateIsolateData(isolate_, uv_default_loop());
   Argv argv{"node", "-e", ";"};
-  auto env = Environment:CreateEnvironment(isolateData, context, 1, *argv, 2, *argv);
-  node::AtExit(at_exit_callback);
+  auto env = node::CreateEnvironment(isolateData, context, 1, *argv, 2, *argv);
+  node::AtExit(env, at_exit_callback);
   node::RunAtExit(env);
   EXPECT_TRUE(called_cb);
 }