Update Cloud SQL high availability flag documentation (#3477) (#2040)

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/3477.txt

@@ -0,0 +1,3 @@
+```release-note:none
+
+```

website/docs/r/sql_database_instance.html.markdown

@@ -240,8 +240,7 @@ The required `settings` block supports:
     for information on how to upgrade to Second Generation instances.
     A list of Google App Engine (GAE) project names that are allowed to access this instance.
 
-* `availability_type` - (Optional) This specifies whether a PostgreSQL instance
-    should be set up for high availability (`REGIONAL`) or single zone (`ZONAL`).
+* `availability_type` - (Optional) The availability type of the Cloud SQL instance, high availability (`REGIONAL`) or single zone (`ZONAL`).'
 
 * `crash_safe_replication` - (Optional, Deprecated) This property is only applicable to First Generation instances.
     First Generation instances are now deprecated, see [here](https://cloud.google.com/sql/docs/mysql/upgrade-2nd-gen)