Docs: Refer to swiftly in CONTRIBUTING.md

The Swift toolchain installation on swift.org indicate to use swiftly to
install the toolchain on macOS and Linux. Update the CONTRIBUTING.md
file to refer to swiftly for toolchain management.

Author: bkhouri

CONTRIBUTING.md

@@ -88,7 +88,7 @@ $> swift --version
 Apple Swift version 5.3
 ```
 
-Note: Alternatively use tools like [swiftenv](https://github.com/kylef/swiftenv) that help manage toolchains versions.
+Note: Alternatively use tools like [swiftly](https://www.swift.org/swiftly/documentation/swiftlydocs/) that help manage toolchains versions.
 
 ## Local Development