Make error message clearer that :on requires a symbol, not a string

The validation added in 5a3dc8092d19c816b0b1203945639cb91d065847 will
reject values for the `:on` option for after_commit and after_rollback
callbacks that are string values like `"create"`.

However, the error message says ":on conditions for after_commit and
after_rollback callbacks have to be one of create,destroy,update". That
looks like a string value *would* be valid.

This commit changes the error message to say ":on conditions for
after_commit and after_rollback callbacks have to be one of [:create,
:destroy, :update]", making it clearer that symbols are required.
Merge pull request #17964 from carols10cents/improve-after-commit-argumenterror-message

Make error message clearer that :on requires a symbol, not a string