Documentation/PCI/msi-howto.rst referred to pci_irq_alloc_vectors() when it
should refer to pci_alloc_irq_vectors().  Fix the typo.

Link: https://lore.kernel.org/r/20191230131428.1200-1-yuzenghui@huawei.comSigned-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>