add missing valid rkey char

by [the rkey spec](https://atproto.com/specs/record-key#record-key-syntax):

> restricted to a subset of ASCII characters — the allowed characters are alphanumeric (A-Za-z0-9), period, dash, underscore, colon, or tilde (.-_:~)

tilde was missing from this check, which leads to at least [jetstream becoming unhappy](https://github.com/bluesky-social/jetstream/issues/54) over valid-but-uncommon rkeys.

this change aligns the check with the spec.

mst/mst_util.go

···
 			continue
 		}
 		switch b {
-		case '_', ':', '.', '-':
+		case '_', ':', '.', '-', '~':
 			continue
 		default:
 			return false