docs/core-api: swiotlb: fix typos

docs/core-api: swiotlb: fix typos

Correct references to swiotlb_tbl_map_single() and
swiotlb_tbl_unmap_single() in the documentation for swiotlb.

Fix two small typos that went unnoticed in commit c93f261dfc39
("Documentation/core-api: add swiotlb documentation"):
swiotlb_tlb_map_single --> swiotlb_tbl_map_single
swiotbl_tlb_unmap_single --> swiotlb_tbl_unmap_single

Signed-off-by: Eder Zulian <ezulian@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241014224026.1838525-1-ezulian@redhat.com