[DOC] ExponentialSmoothing default method change from L-BFGS-B to SLS…

…QP (#6186)

#### Reference Issues/PRs
While exploring #6175, realized that the default parameter for `method`
in Exponential Smoothing should be `SLSQP`. This also affects
statsmodels documentation and there is already an [open
PR](statsmodels/statsmodels#9129) in their repo
to fix this.

#### What does this implement/fix? Explain your changes.
Changes docs of default `method='L-BFGS-B'` to `method='SLSQP'`.

.all-contributorsrc

@@ -2706,6 +2706,15 @@
       "contributions": [
         "doc"
       ]
+    },
+    {
+      "login": "manuel-munoz-aguirre",
+      "name": "Manuel Muñoz Aguirre",
+      "avatar_url": "https://avatars.githubusercontent.com/u/5576458?v=4",
+      "profile": "https://github.com/manuel-munoz-aguirre",
+      "contributions": [
+        "doc"
+      ]
     }
   ]
 }

sktime/forecasting/exp_smoothing.py

@@ -70,7 +70,7 @@ class ExponentialSmoothing(_StatsModelsAdapter):
         starting values are determined using a combination of grid search
         and reasonable values based on the initial values of the data. See
         the notes for the structure of the model parameters.
-    method : str, default "L-BFGS-B"
+    method : str, default "SLSQP"
         The minimizer used. Valid options are "L-BFGS-B" , "TNC",
         "SLSQP" (default), "Powell", "trust-constr", "basinhopping" (also
         "bh") and "least_squares" (also "ls"). basinhopping tries multiple