Change the error msg in jwtvendorTests too

Signed-off-by: Ryan Liang <jiallian@amazon.com>

Author: RyanL1997

src/test/java/org/opensearch/security/authtoken/jwt/JwtVendorTest.java

@@ -47,7 +47,7 @@ public void testCreateJwkFromSettingsWithoutSigningKey() {
             }
         });
         Assert.assertEquals(
-            "java.lang.Exception: Settings for key is missing. Please specify at least the option signing_key with a shared secret.",
+            "java.lang.Exception: Settings for signing key is missing. Please specify at least the option signing_key with a shared secret.",
             exception.getMessage()
         );
     }