Align columns of the first tabular

[emollier]

Greetings,

There are three tabulars in doc/manual.rst, but only two are rendered in various contexts:

• sphinx-build -b man,
• rst2man,
• the Github renderer at: https://github.com/PacificBiosciences/kineticsTools/blob/master/doc/manual.rst

This slight modification should bring it back to rendered documentations.

Kind Regards, Étienne.