fix hyphen (#378)

This uses an em dash instead of double lines (--)

plugin/http/Error.lua

@@ -8,7 +8,7 @@ Error.Kind = {
 	},
 	ConnectFailed = {
 		message = "Couldn't connect to the Rojo server.\n" ..
-			"Make sure the server is running -- use 'rojo serve' to run it!",
+			"Make sure the server is running — use 'rojo serve' to run it!",
 	},
 	Timeout = {
 		message = "HTTP request timed out.",
@@ -63,4 +63,4 @@ function Error.fromRobloxErrorString(message)
 	return Error.new(Error.Kind.Unknown, message)
 end
 
-return Error
+return Error