docs: nvme: fix grammar in nvme-pci-endpoint-target.rst

Notable changes:

 - Use "an NVMe" instead of "a NVMe" throughout the document
 - Fix incorrect phrasing such as "will is discoverable" -> "is
   discoverable"
 - Ensure consistent and proper article usage for clarity.

Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>