Sync string in EXPLAIN code with phrasing of manual (subquery, not subselect).

sql/sql_select.cc

@@ -8100,9 +8100,9 @@ int mysql_explain_union(THD *thd, SELECT_LEX_UNIT *unit, select_result *result)
 			       ((sl == first)?
 				((sl->linkage == DERIVED_TABLE_TYPE) ?
 				 "DERIVED":
-				((sl->dependent)?"DEPENDENT SUBSELECT":
-				 (sl->uncacheable?"UNCACHEABLE SUBSELECT":
-				   "SUBSELECT"))):
+				((sl->dependent)?"DEPENDENT SUBQUERY":
+				 (sl->uncacheable?"UNCACHEABLE SUBQUERY":
+				   "SUBQUERY"))):
 				((sl->dependent)?"DEPENDENT UNION":
 				 sl->uncacheable?"UNCACHEABLE UNION":
 				  "UNION"))),