fix did multi docstring example

Author: SvenKlaassen

doubleml/did/did_multi.py

@@ -134,7 +134,22 @@ class DoubleMLDIDMulti:
     ...     gt_combinations="standard",
     ...     control_group="never_treated",
     ... )
-    >>> print(dml_did_obj.fit())
+    >>> print(dml_did_obj.fit().summary)
+                                      coef   std err  ...     2.5 %    97.5 %
+    ATT(2025-03,2025-01,2025-02) -0.797617  0.459617  ... -1.698450  0.103215
+    ATT(2025-03,2025-02,2025-03)  0.270311  0.456453  ... -0.624320  1.164941
+    ATT(2025-03,2025-02,2025-04)  0.628213  0.895275  ... -1.126494  2.382919
+    ATT(2025-03,2025-02,2025-05)  1.281360  1.327121  ... -1.319750  3.882470
+    ATT(2025-04,2025-01,2025-02) -0.078095  0.407758  ... -0.877287  0.721097
+    ATT(2025-04,2025-02,2025-03)  0.223625  0.479288  ... -0.715764  1.163013
+    ATT(2025-04,2025-03,2025-04)  1.008674  0.455564  ...  0.115785  1.901563
+    ATT(2025-04,2025-03,2025-05)  2.941047  0.832991  ...  1.308415  4.573679
+    ATT(2025-05,2025-01,2025-02) -0.102282  0.454129  ... -0.992359  0.787795
+    ATT(2025-05,2025-02,2025-03)  0.108742  0.547794  ... -0.964914  1.182399
+    ATT(2025-05,2025-03,2025-04)  0.253610  0.422984  ... -0.575423  1.082643
+    ATT(2025-05,2025-04,2025-05)  1.264255  0.487934  ...  0.307923  2.220587
+    <BLANKLINE>
+    [12 rows x 6 columns]
     """
 
     def __init__(