Fix MixedModelMatrices doctsrings

Author: jsakv

causalpy/formula.py

@@ -110,6 +110,13 @@ class MixedModelMatrices:
 
     Examples
     --------
+    >>> data = pd.DataFrame(
+    ...     {
+    ...         "y": [0.1, -0.2, 0.3, 0.0],
+    ...         "x1": [1.0, 0.0, 1.0, 0.5],
+    ...         "store_id": ["s1", "s1", "s2", "s2"],
+    ...     }
+    ... )
     >>> mm = parse_formula("y ~ 1 + x1 + (1 + x1 | store_id)", data)
     >>> mm.lhs.shape[0] == mm.rhs.shape[0] == mm.Z.shape[0]
     True