provider/google: Mention PostgreSQL in sql_database_instance docs

As mentioned in hashicorp#12617 the
google_sql_database_instance resource already supports `POSTGRES_9_6` as
`database_version`, it is just undocumented so far.

website/source/docs/providers/google/r/sql_database_instance.html.markdown

@@ -44,7 +44,7 @@ The following arguments are supported:
 - - -
 
 * `database_version` - (Optional, Default: `MYSQL_5_6`) The MySQL version to
-    use. Can be either `MYSQL_5_6` or `MYSQL_5_7` for second-generation
+    use. Can be `MYSQL_5_6`, `MYSQL_5_7` or `POSTGRES_9_6` for second-generation
     instances, or `MYSQL_5_5` or `MYSQL_5_6` for first-generation instances.
     See Google's [Second Generation Capabilities](https://cloud.google.com/sql/docs/1st-2nd-gen-differences)
     for more information.