Linktype check error message becomes more correct

The type linkType has String() method and the error message is created
using it.

Bug: 180477804
Test: m nothing
Change-Id: I74fe9c93b74904177dbe9d29cd3aa3304b67ba4f
diff --git a/apex/apex_test.go b/apex/apex_test.go
index 181946b..bbc4b93 100644
--- a/apex/apex_test.go
+++ b/apex/apex_test.go
@@ -2039,7 +2039,7 @@
 		},
 		{
 			name:          "Updatable apex with non-stable transitive dep",
-			expectedError: "compiles against Android API, but dependency \"transitive-jar\" is compiling against non-public Android API.",
+			expectedError: "compiles against Android API, but dependency \"transitive-jar\" is compiling against private API.",
 			bp: `
 				apex {
 					name: "myapex",