doc/rados: add math markup to placement-groups.rst

doc/rados: add math markup to placement-groups.rst

Add some math markup to some of the mathematical expressions in
doc/rados/operations/placement-groups.rst.

This change has advantages as well as disadvantages. On the one hand,
the mathematical expressions look nicer and are easier to read.  On the
other hand, the rendered mathematical expressions are not searchable as
a part of the text, which makes CTRL+F searches useless in finding them.

This PR is offered as an RFC on the matter of whether mathematical
expressions should be rendered with the :math: extension.

Signed-off-by: Zac Dover <zac.dover@proton.me>