powerpc/perf: Fix spelling of "its"

Use the possessive "its" instead of the contraction of "it is" (it's).

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211223003942.22098-1-rdunlap@infradead.org