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

Reported-by: Stephen Rothwell
Tested-by: Stephen Rothwell # build test
Signed-off-by: Aneesh Kumar K.V
Signed-off-by: Michael Ellerman
Link: https://lore.kernel.org/r/20210825042447.106219-1-aneesh.kumar@linux.ibm.com