Prefer HTTPS protocol in README (#28)

Author: rinsuki

README.md

@@ -11,7 +11,7 @@ The markup tree provided by this package is comprised of immutable/persistent, t
 In your `Package.swift` Swift Package Manager manifest, add the following dependency to your `dependencies` argument:
 
 ```swift
-.package(url: "ssh://git@github.com/apple/swift-markdown.git", .branch("main")),
+.package(url: "https://github.com/apple/swift-markdown.git", .branch("main")),
 ```
 
 Add the dependency to any targets you've declared in your manifest: