cmd/gomobile: be consistent about how the target flag is spelled

Change-Id: Ic9b66152c4cdc18e843a41818ac3319157ce11ca
Reviewed-on: https://go-review.googlesource.com/23305
Reviewed-by: David Crawshaw <crawshaw@golang.org>
diff --git a/cmd/gomobile/build_iosapp.go b/cmd/gomobile/build_iosapp.go
index aae72c6..0b2a923 100644
--- a/cmd/gomobile/build_iosapp.go
+++ b/cmd/gomobile/build_iosapp.go
@@ -20,7 +20,7 @@
 func goIOSBuild(pkg *build.Package) (map[string]bool, error) {
 	src := pkg.ImportPath
 	if buildO != "" && !strings.HasSuffix(buildO, ".app") {
-		return nil, fmt.Errorf("-o must have an .app for target=ios")
+		return nil, fmt.Errorf("-o must have an .app for -target=ios")
 	}
 
 	productName := rfc1034Label(path.Base(pkg.ImportPath))