`copilot-theorem`: Look for `falsifiable` (not `invalid`). Refs #495.

[RyanGlScott]

In Kind2-0.7.2, disproven properties are tagged with falsifiable in the XML output, but the code in copilot-theorem's Kind2 backend was instead searching for a tag named invalid. As a result, copilot-theorem would error when attempting to disprove properties that are false, as it fail to parse the XML output. This fixes the issue by replacing invalid with falsifiable.

Fixes #495.