minor: update compatibility (#1303)

Leftover from #1281

Author: kazuyukitanimura

docs/source/user-guide/compatibility.md

@@ -133,7 +133,6 @@ The following cast operations are generally compatible with Spark except for the
 | string | date | Only supports years between 262143 BC and 262142 AD |
 | date | string |  |
 | timestamp | long |  |
-| timestamp | decimal |  |
 | timestamp | string |  |
 | timestamp | date |  |