cmd/xurls: reject unexpected -fix string values

While here, tweak the help text and run -fix itself on the README.

README.md

@@ -18,8 +18,8 @@ func main() {
 }
 ```
 
-Since API is centered around [regexp.Regexp](https://golang.org/pkg/regexp/#Regexp),
-many other methods are available, such as finding the [byte indexes](https://golang.org/pkg/regexp/#Regexp.FindAllIndex)
+Since API is centered around [regexp.Regexp](https://pkg.go.dev/regexp#Regexp),
+many other methods are available, such as finding the [byte indexes](https://pkg.go.dev/regexp#Regexp.FindAllIndex)
 for all matches.
 
 The regular expressions are compiled when the API is first called.

cmd/xurls/main.go

@@ -47,7 +47,7 @@ func init() {
 	flag.Var(&fix, "fix", "")
 	flag.Usage = func() {
 		fmt.Fprint(os.Stderr, `
-Usage: xurls [-h] [files]
+Usage: xurls [flags] [files]
 
 xurls extracts urls from text using regular expressions.
 If no files are given, it reads from standard input.
@@ -230,6 +230,9 @@ func main1() int {
 	case "auto", "all": // enabled via -fix=auto, -fix=all, etc
 	case "true": // enabled via -fix; normalize
 		fix = "auto"
+	default:
+		flag.Usage()
+		return 1
 	}
 	var re *regexp.Regexp
 	if *relaxed {