Update resource_grant import description (#196)

Fix #194

Author: bigwheel

website/docs/r/grant.html.markdown

@@ -88,14 +88,14 @@ No further attributes are exported.
 ## Import
 
 Grants can be imported using user, host, database and table.
-For grants without explicit database or tables, leave these fields empty.
+For grants without explicit database or tables, use `*`.
 
 You can also add an extra at sign `@` to the import definition to specify
 the grant contains WITH GRANT OPTION.
 
 ```
 $ terraform import mysql_grant.example user@host@database@table
-$ terraform import mysql_grant.without_db user@host@@
+$ terraform import mysql_grant.all_db user@host@*@*
 
 # Import the first example with grant option
 $ terraform import mysql_grant.example user@host@database@table@