cmd/go/internal/modfile: make error message for module path more clear

Fixes #31775

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Change-Id: I59c4e90f20d1b31161c259680b48b7be7218bf58
Reviewed-on: https://go-review.googlesource.com/c/go/+/175017
Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modfile/rule.go

@@ -195,7 +195,7 @@ func (f *File) add(errs *bytes.Buffer, line *Line, verb string, args []string, f
 		f.Module = &Module{Syntax: line}
 		if len(args) != 1 {
 
-			fmt.Fprintf(errs, "%s:%d: usage: module module/path [version]\n", f.Syntax.Name, line.Start.Line)
+			fmt.Fprintf(errs, "%s:%d: usage: module module/path\n", f.Syntax.Name, line.Start.Line)
 			return
 		}
 		s, err := parseString(&args[0])