Rename this_work_item::get_work_group()

get_group() conveys that it returns a sycl::group, avoids introducing a
dependency on the khr_group_interface extension, and reserves get_work_group()
for future use.

adoc/extensions/sycl_khr_work_item_queries.adoc

@@ -72,13 +72,13 @@ in the [code]#sycl::nd_range#.
 
 '''
 
-.[apidef]#khr::this_work_item::get_work_group#
-[source,role=synopsis,id=api:khr-this-work-item-get-work-group]
+.[apidef]#khr::this_work_item::get_group#
+[source,role=synopsis,id=api:khr-this-work-item-get-group]
 ----
 namespace sycl::khr::this_work_item {
 
 template <int Dimensions>
-group<Dimensions> get_work_group();
+group<Dimensions> get_group();
 
 }
 ----