Fix repl-executable for Swift 5.7

Swift 5.5: `repl` is not mentioned in the manual but is ignored silently.
Swift 5.6: `repl` is the default subcommand.
Swift 5.7: `repl` is not default.

Author: taku0

swift-mode-repl.el

@@ -43,7 +43,7 @@
   :group 'swift)
 
 (defcustom swift-mode:repl-executable
-  (concat (when (executable-find "xcrun") "xcrun ") "swift")
+  (concat (when (executable-find "xcrun") "xcrun ") "swift repl")
   "Path to the Swift CLI.  The string is split by spaces, then unquoted."
   :tag "Swift Mode REPL Executable"
   :type '(choice string (list string))