fix link to public-server mailing list

[bgruening]

(Please replace this header with a description of your pull request. Please include BOTH what you did and why you made the changes. The "why" may simply be citing a relevant Galaxy issue.)
(If fixing a bug, please add any relevant error or traceback)
(For UI components, it is recommended to include screenshots or screencasts)

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.