Fix running test that validates manifest-based DAGs (#619)

When checking the latest test runs in several branches
for**Run-Integration-Tests (3.8, 2.7)**, example:

https://github.com/astronomer/astronomer-cosmos/actions/runs/6612130380/job/17957443769?pr=605#logs

I noticed that `cosmos_manifest_example` was not being run. This PR
solves this issue.

(cherry picked from commit 9056125)

pyproject.toml

@@ -175,8 +175,7 @@ test-integration = """pytest -vv \
 --cov-report=xml \
 --durations=0 \
 -m integration  \
--k 'not (sqlite or example_cosmos_sources or example_cosmos_python_models or example_virtualenv or cosmos_manifest_example)'
-"""
+-k 'not (sqlite or example_cosmos_sources or example_cosmos_python_models or example_virtualenv)'"""
 test-integration-expensive = """pytest -vv \
 --cov=cosmos \
 --cov-report=term-missing \