docs: powerpc: Fix misspellings and grammar errors

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: He Ying <heying24@huawei.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210326100853.173586-1-heying24@huawei.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>