docs: remove the explicit file name from edit url

[piksel]

The current edit buttons in the doc adds an additional index.md to the edit URLs:

https://github.com/containrrr/watchtower/edit/main/docs/index.md/index.md

This just removes the explicit file name to make the URLs relative to the repo default branch.

Follow up from #846

[codecov]

Codecov Report

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@@           Coverage Diff           @@
##             main     #879   +/-   ##
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  Coverage        ?   43.07%           
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  Files           ?       24           
  Lines           ?     1379           
  Branches        ?        0           
=======================================
  Hits            ?      594           
  Misses          ?      724           
  Partials        ?       61           

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