Documentation: Fix path for NVMe PCI endpoint target driver

The path for the driver points to an non-existent file.  Update path with
the correct file: drivers/nvme/target/pci-epf.c

Signed-off-by: Rick Wertenbroek <rick.wertenbroek@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://patch.msgid.link/20250423095643.490495-1-rick.wertenbroek@gmail.com