[TDF] Avoid using the same name for two custom columns in tutorial

Redefinition of a custom column is currently allowed due to lack
of checking and sort of works (last definition "wins"), but will
be checked for and will count as an error in the future.

tutorials/dataframe/tdf001_introduction.C

@@ -155,7 +155,7 @@ int tdf001_introduction()
    // Additional columns can be expressed as strings. The content must be C++
    // code. The name of the variables must be the name of the branches. The code
    // is just in time compiled.
-   auto entries_sum2 = d.Define("sum", "b1 + b2").Filter("sum > 4.2").Count();
+   auto entries_sum2 = d.Define("sum2", "b1 + b2").Filter("sum2 > 4.2").Count();
    std::cout << *entries_sum2 << std::endl;
 
    return 0;