Track lower-case maxtext package in coverage

The NNX-migrated code lives in src/maxtext/ (lower-case) but
.coveragerc only listed the upper-case MaxText package in
[run] source. coverage.py therefore never instrumented the
new files, so added lines showed up on Codecov with no
coverage data and suppressed patch coverage.

The existing [paths] aliasing only merges already-collected
data across filesystems; it does not control what is traced
in the first place.

Author: ecnal-cienet

.coveragerc

@@ -2,6 +2,7 @@
 
 [run]
 source = MaxText
+         maxtext
 branch = True
 omit =
     tests/*