doc: fix formatting on PG recommendation

Previous commit (047287afbe0ddfaaafd05e9dbf25c1c7dea9a1be) broke
formatting on the formula, and also made mixed formula and text oddly,
which on second thought didn't look too good.

Add the note about the power of two to the following paragraph
instead, in prose.

Signed-off-by: Florian Haas <florian@hastexo.com>
Merge pull request #1443 from fghaas/doc-fix

doc: fix formatting on PG recommendation

Reviewed-by: Sage Weil <sage@inktank.com>