fix at_time notebook links

- The "source lag" wiki link was replaced with a link to the FAQ doc in
  our API developer docs (fixed in #296)
- The "how do I see previous values for a time series point to see how
  the value changed over time" link was replaced with a link to our
  docs for the show_revisions option for get_data_points. (removed in
  #296, adding back correct replacement link)

Author: john li

api/client/samples/at-time-query-examples.ipynb

@@ -47,7 +47,8 @@
     "--------\n",
     "- https://developers.gro-intelligence.com/api.html#groclient.GroClient.get_data_points\n",
     "- https://developers.gro-intelligence.com/api.html#groclient.GroClient.lookup\n",
-    "- https://developers.gro-intelligence.com/faq.html#what-does-sourcelag-mean-when-i-use-client-lookup-to-inspect-a-source-s-details"
+    "- https://developers.gro-intelligence.com/faq.html#what-does-sourcelag-mean-when-i-use-client-lookup-to-inspect-a-source-s-details\n",
+    "- https://developers.gro-intelligence.com/querying-data.html#show-revisions"
    ]
   },
   {