Use HTTP Types v1.4.0 (#99)

HTTP Types introduced a breaking changes in v1.5.0
apple/swift-http-types#107

This PR pin the dependency to v1.4.0

Author: sebsto

Package.swift

@@ -11,7 +11,7 @@ let package = Package(
     ],
     dependencies: [
         .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
-        .package(url: "https://github.com/apple/swift-http-types.git", from: "1.0.0"),
+        .package(url: "https://github.com/apple/swift-http-types.git", exact: "1.4.0"),
     ],
     targets: [
         .target(