Drop experimenal from generate-manual (#475)

- Removes the experimental prefix from the generate-manual plugin. The
  manual generation still is not mature, but we would like to avoid
  breaking adopters scripts by delaying changing the plugin's name in
  the future.

Package@swift-5.6.swift

@@ -43,7 +43,7 @@ var package = Package(
             name: "GenerateManualPlugin",
             capability: .command(
                 intent: .custom(
-                    verb: "experimental-generate-manual",
+                    verb: "generate-manual",
                     description: "Generate a manual entry for a specified target.")),
             dependencies: ["generate-manual"]),