Fix the Towncrier philosophy link

[Jamim]

What do these changes do?

These changes fix a broken link to towncrier's philosophy.

Are there changes in behavior for the user?

Link from CHANGES/README.rst to towncrier's philosophy will be fixed.

Related issue number

Similar PR:

• aio-libs/frozenlist#574

Checklist

• [x] I think the code is well written
• [x] Documentation reflects the changes

Best regards!

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (cfdde40) 99.53% compared to head (4f2d714) 99.53%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #439 +/- ## ======================================= Coverage 99.53% 99.53% ======================================= Files 4 4 Lines 427 427 Branches 45 45 ======================================= Hits 425 425 Misses 1 1 Partials 1 1 ```

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