That was likely a temporary bug with HTTPRedirect. That code seems to be working now. However, I would suggest editing it to use the name of the input variable. I think much easier to read/understand:
CloudDeploy[
FormFunction[{"Website" -> <|"Interpreter" -> "SemanticURL", "Input" -> "Wikipedia"|>},
HTTPRedirect[#Website] &,
AppearanceRules -> <|"Title" -> "WebGo!",
"Description" -> "Write the name of any website, and we will take you there!",
"SubmitLabel" -> "Go!"|>]]