Correct bit numbers in `PCMR` documentation.
Correct bit numbers in `PCMR` documentation.

These numbers were inverted in the documentation. This commit resolves #15.

lowRISC/ibex,AmbiML/ibex,AmbiML/ibex,AmbiML/ibex,lowRISC/ibex,lowRISC/ibex,AmbiML/ibex,lowRISC/ibex