[gardening]: update macos architecture wording in README build state matrix

the current macos CI builds for both x86 and arm, but the README
suggested that it was only x86. update the terminology to make things a
bit more clear.

Author: jamieQ

README.md

@@ -8,7 +8,7 @@
 
 | | **Architecture** | **Build** |
 |---|:---:|:---:|
-| **macOS**        | x86_64 |[![Build Status](https://ci.swift.org/job/oss-swift-package-macos/lastCompletedBuild/badge/icon)](https://ci.swift.org/job/oss-swift-package-macos)|
+| **macOS**        | Universal |[![Build Status](https://ci.swift.org/job/oss-swift-package-macos/lastCompletedBuild/badge/icon)](https://ci.swift.org/job/oss-swift-package-macos)|
 | **Ubuntu 20.04** | x86_64 |[![Build Status](https://ci.swift.org/job/oss-swift-package-ubuntu-20_04/lastCompletedBuild/badge/icon)](https://ci.swift.org/job/oss-swift-package-ubuntu-20_04)|
 | **Ubuntu 20.04** | AArch64 |[![Build Status](https://ci.swift.org/job/oss-swift-package-ubuntu-20_04-aarch64/lastCompletedBuild/badge/icon)](https://ci.swift.org/job/oss-swift-package-ubuntu-20_04-aarch64)|
 | **Ubuntu 22.04** | x86_64 |[![Build Status](https://ci.swift.org/job/oss-swift-package-ubuntu-22_04/lastCompletedBuild/badge/icon)](https://ci.swift.org/job/oss-swift-package-ubuntu-22_04)|