update the part to Google+ API

What does this MR do?

Our docs mention the following path to enable Google+ API:

From the Dashboard select ENABLE APIS AND SERVICES > Compute > Google+ API > Enable

but in fact it is:

ENABLE APIS AND SERVICES > Social > Google+ API > Enable

Reference: https://console.cloud.google.com/apis/library/plus.googleapis.com

This MR is created to update this information in the docs.