hacking.md: Update instructions to force OS version (#29770)

Update instructions to force OS version to reflect Homebrew/brew#1883.

doc/development/hacking.md

@@ -59,14 +59,12 @@ This form can also be combined with a specific Ruby interpreter as above.
 
 ### Forcing a Specific macOS Release
 
-The environment variable `$MACOS_RELEASE` can be overridden at the command line for test purposes:
+The environment variable `$MACOS_VERSION` can be overridden at the command line for test purposes:
 
 ```bash
-$ MACOS_RELEASE=10.9 brew cask info <cask>
+$ MACOS_VERSION=10.9 brew cask info <cask>
 ```
 
-The environment variable `$MACOS_RELEASE_WITH_PATCHLEVEL` is also available, though not consulted directly. Use `$MACOS_RELEASE` for testing.
-
 ### Target Ruby Versions
 
 Homebrew-Cask requires a Ruby interpreter version 2.0 or above. This is the default system Ruby on Mavericks (10.9) and later.