fixed naming of variable

Author: Anirav Kareddy

src/test/java/software/amazon/encryption/s3/materials/MaterialsDescriptionTest.java

@@ -251,7 +251,7 @@ public void testRsaMaterialsDescriptionInInstructionFile() {
       .build();
 
     final String input = "Testing Materials Description in Instruction File!";
-    final String objectKey = "test-rsa-materials-description-in-object-metadata";
+    final String objectKey = "test-rsa-materials-description-in-instruction-file";
 
     client.putObject(builder -> builder
       .bucket(BUCKET)