added semicolon (;) to improve warning message clarity

added semicolon (;) to improve warning message clarity

### Description

Added a semicolon to improve the clarity of warning message.  I actually had a table named `backend`, and thought it was involved!

While updating the code, I noticed no test that directly tests for this warning message. There are tests for the `Can't sort tables for DROP;` prefix of this message and the `exc.CircularDependencyError`; and some tests for the `exc.CircularDependencyError` message itself. I couldn't find any test for this particular message though.  (Just thought I'd bring that up)

No issue created, because this is minor.

Closes: #5431
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5431
Pull-request-sha: 87fb5733ff4fc1a13dd94277716814ea852f654c

Change-Id: I87a504d30a7dd5155c34f7d7f30b2116d0d3cd3f