tests: fix a typo in `desc()` revset in test-merge-criss-cross.t
I was looking into fixing the broken behavior only to find out that we had a
typo in the commit message and merge was not performed.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.