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.
#require black test-repo
$ cd $RUNTESTDIR/..
$ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`