powerpc/doc: Fix htmldocs errors

Fix make htmldocs related errors with the newly added associativity.rst
doc file.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au> # build test
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210825042447.106219-1-aneesh.kumar@linux.ibm.com