trivial: fix typo in bio_alloc kernel doc

 *

 *   If %__GFP_WAIT is set, then bio_alloc will always be able to allocate

 *   a bio. This is due to the mempool guarantees. To make this work, callers

 *   must never allocate more than 1 bio at the time from this pool. Callers

 *   must never allocate more than 1 bio at a time from this pool. Callers

 *   that need to allocate more than 1 bio must always submit the previously

 *   allocate bio for IO before attempting to allocate a new one. Failure to

 *   allocated bio for IO before attempting to allocate a new one. Failure to

 *   do so can cause livelocks under memory pressure.

 *

 **/