[Cmake] Wrong help message for TCL/TK_LIBRARY

[Ken Martin]

> Just a question a question : In english, do you say ?
>          Where can one the python, python20 or python21
> libraries be found
> or
>          Where can one the python, python20 or python21
> library be found

Here are a couple options:

Where can the python, python20 or python21 library be found

or

Where can one of the python, python20 or python21 libraries be found

The first works well also for a single library, the second really only
sounds correct for multiple libraries.