Update models.rst

Author: SvenKlaassen

doc/guide/models.rst

@@ -237,7 +237,7 @@ Estimation is conducted via its ``fit()`` method:
     ml_m = RandomForestClassifier(n_estimators=100, max_depth=5, min_samples_leaf=5)
     np.random.seed(42)
     data = make_did_SZ2020(n_obs=500, return_type='DataFrame') 
-    # y is already defined a the difference of observed outcomes
+    # y is already defined as the difference of observed outcomes
     obj_dml_data = dml.DoubleMLData(data, 'y', 'd')
     dml_did_obj = dml.DoubleMLDID(obj_dml_data, ml_g, ml_m)
     print(dml_did_obj.fit())