powerpc/doc: Fix htmldocs errors

powerpc/doc: Fix htmldocs errors

Origin: https://git.kernel.org/linus/f50da6edbf1ebf35dd8070847bfab5cb988d472b

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
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name powerpc-doc-Fix-htmldocs-errors.patch