Fix type name in range field query string tests

This came up after a backport, this commit fixes the type name used in
the range field query string tests.

Author: jasontedor

core/src/test/java/org/elasticsearch/index/mapper/RangeFieldQueryStringQueryBuilderTests.java

@@ -54,7 +54,7 @@ public class RangeFieldQueryStringQueryBuilderTests extends AbstractQueryTestCas
 
     @Override
     protected void initializeAdditionalMappings(MapperService mapperService) throws IOException {
-        mapperService.merge("_doc", new CompressedXContent(PutMappingRequest.buildFromSimplifiedDef("_doc",
+        mapperService.merge("doc", new CompressedXContent(PutMappingRequest.buildFromSimplifiedDef("doc",
             INTEGER_RANGE_FIELD_NAME, "type=integer_range",
             LONG_RANGE_FIELD_NAME, "type=long_range",
             FLOAT_RANGE_FIELD_NAME, "type=float_range",