cmd/go: fix typo

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/138700043

src/cmd/go/test.go

@@ -1059,7 +1059,7 @@ func (b *builder) notest(a *action) error {
 	return nil
 }
 
-// isTestMain tells whether fn is a TestMain(m *testing.Main) function.
+// isTestMain tells whether fn is a TestMain(m *testing.M) function.
 func isTestMain(fn *ast.FuncDecl) bool {
 	if fn.Name.String() != "TestMain" ||
 		fn.Type.Results != nil && len(fn.Type.Results.List) > 0 ||