update module docstring

Author: drbenvincent

causalpy/pymc_models.py

@@ -11,7 +11,7 @@
 #   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
-"""PyMC models"""
+"""Custom PyMC models for causal inference"""
 
 from typing import Any, Dict, Optional