xsk: Fix a documentation mistake in xsk_queue.h

After 'peeking' the ring, the consumer, not the producer, reads the data.
Fix this mistake in the comments.

Fixes: 15d8c916 ("xsk: Add function naming comments and reorder functions")
Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20200928082344.17110-1-ciara.loftus@intel.com