share/doc/homebrew/README: fix supporters link.

The link to the Kickstarter Supporters file was broken. This new link
points to the ./Kickstarter-Supporters.md file.

Closes Homebrew/homebrew#50335.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

share/doc/homebrew/README.md

@@ -30,4 +30,4 @@ Read [CONTRIBUTING.md](/.github/CONTRIBUTING.md).
 
 ## Supporters
 [A list of the awesome people who gave £5 or more to our
-Kickstarter](https://github.com/Homebrew/homebrew/blob/master/SUPPORTERS.md).
+Kickstarter](./Kickstarter-Supporters.md).