python3Packages.xarray.optional-dependencies.complete: fix the eval

Without the change the eval fails as:

    $ nix-instantiate -A pypy310Packages.xarray.optional-dependencies.complete
    error:
       … while evaluating a path segment
         at pkgs/development/python-modules/xarray/default.nix:96:27:
           95|     ];
           96|     complete = with self; accel ++ io ++ etc ++ parallel + viz;
             |                           ^
           97|   });

       error: cannot coerce a list to a string: [ «thunk» «thunk» «thunk» «thunk» «thunk» «thunk» «thunk» «thunk» «thunk» «thunk» «13 items elided» ]

This happens because of `+` concat operator instead of `++`.

pkgs/development/python-modules/xarray/default.nix

@@ -93,7 +93,7 @@ buildPythonPackage rec {
       # nc-time-axis
       seaborn
     ];
-    complete = with self; accel ++ io ++ etc ++ parallel + viz;
+    complete = with self; accel ++ io ++ etc ++ parallel ++ viz;
   });
 
   nativeCheckInputs = [