[SYCL][Doc] Fix namespace in forward_progress spec (intel#12874)

An automated renaming led to the sycl part of the namespace appearing in
the wrong place.

Signed-off-by: John Pennycook <[email protected]>

Author: Pennycook

sycl/doc/extensions/proposed/sycl_ext_oneapi_forward_progress.asciidoc

@@ -254,7 +254,7 @@ of forward progress guarantee defined in the {cpp} core language specification.
 
 [source,c++]
 ----
-namespace ext::oneapi::experimental::sycl {
+namespace sycl::ext::oneapi::experimental {
 
 enum class forward_progress_guarantee {
   concurrent,
@@ -279,7 +279,7 @@ execution within a SYCL implementation.
 
 [source,c++]
 ----
-namespace ext::oneapi::experimental::sycl {
+namespace sycl::ext::oneapi::experimental {
 
 enum class execution_scope {
   work_item,