[doc] tutorial: use alchemlyb.concat (#399)

replace `pandas.concat()` with `alchemlyb.concat()` in the tutorial (given that we explicitly tell users to use it)

docs/tutorial.rst

@@ -76,7 +76,7 @@ method. More estimators are available in the section on
     >>> import pandas as pd
 
     >>> mbar = MBAR()
-    >>> mbar.fit(pd.concat(decorrelated_u_nk_list))
+    >>> mbar.fit(alchemlyb.concat(decorrelated_u_nk_list))
     >>> mbar.delta_f_
               0.00      0.25      0.50      0.75      1.00
     0.00  0.000000  1.613595  2.553407  2.983336  3.039517
@@ -152,4 +152,4 @@ also perform a set of analysis that allows the user to examine the quality of
 the estimation.
 
 
-.. _alchemtest: https://github.com/alchemistry/alchemtest
+.. _alchemtest: https://github.com/alchemistry/alchemtest