Fix unit section for consistency with VOUnits

[mbtaylor]

Section 4.4 of VOTable 1.4 references VOUnits as the recommended syntax for the unit attribute, but the summary in the text ("no special symbol is required for a power") and some of the examples ("m2" and "cm-2.s-1.keV-1") are incorrect with respect to the VOUnits standard, which uses "**" for raising to a power. The examples and text, which were presumably left over from pre-VOUnit days, should be updated.

It's also not clear from the text whether VOUnits are REQUIRED or RECOMMENDED; it says

The syntax of the unit string is defined in reference [3]

but also

Reference [3] contains the recommended rules for writing string representations of the unit strings, called VOUnits.

This should be clarified.

[mbtaylor]

This was fixed by PR #51.