Fix config comment to list all analytics providers. (mmistakes#2607)

The main config was properly updated with the google-gtag
feature commit (79d0b75), but was
removed, possibly by bad merge, with an unrelated commit
(bac26e9) two days later.

The docs and test configs appear to have never received the initial
google-gtag update.

Author: smanjo

docs/_config.yml

@@ -81,7 +81,7 @@ social:
 
 # Analytics
 analytics:
-  provider               : "google-universal" # false (default), "google", "google-universal", "custom"
+  provider               : "google-universal" # false (default), "google", "google-universal", "google-gtag", "custom"
   google:
     tracking_id          : "UA-2011187-3" # Replace this with your ID, or delete
     anonymize_ip         : true

test/_config.yml

@@ -75,7 +75,7 @@ social:
 
 # Analytics
 analytics:
-  provider               : false # false (default), "google", "google-universal", "custom"
+  provider               : false # false (default), "google", "google-universal", "google-gtag", "custom"
   google:
     tracking_id          :