Fix test assertion due to change in DerivedFieldMapper CONTENT_TYPE

Signed-off-by: Mohammad Qureshi <qreshi@amazon.com>

server/src/test/java/org/opensearch/index/mapper/DerivedFieldMapperTests.java

@@ -109,7 +109,7 @@ public void testFieldInDerivedAndProperties() throws IOException {
         //  If it didn't fail here, it would hit the MapperParsingException for the field being defined more than once
         //  when MappingLookup is initialized
         assertEquals(
-            "Failed to parse mapping [_doc]: mapper [field] cannot be changed from type [derived_field] to [keyword]",
+            "Failed to parse mapping [_doc]: mapper [field] cannot be changed from type [derived] to [keyword]",
             ex.getMessage()
         );
     }