docs: Fix GithubReleases strategy block example

The `strategy` block example for `GithubReleases` in the
`brew livecheck` documentation should be using `release` inside the
`#map` block instead of `json`. I accidentally overlooked one instance
of `json` when adapting the code from the `GithubLatest` example above
it.

docs/Brew-Livecheck.md

@@ -176,7 +176,7 @@ livecheck do
     json.map do |release|
       next if release["draft"] || release["prerelease"]
 
-      match = json["tag_name"]&.match(regex)
+      match = release["tag_name"]&.match(regex)
       next if match.blank?
 
       match[1]