Finess the fibre distribution text

.. better definition of current classification of
   fibre types, fell short of discussing potential
   it is a continum with staining techniques creating
   an artificial distinction.

.. gave an example distribution to get a sense of the
   bias towards I and IIa in the normal population.
Merge branch 'master' of https://github.com/GoldenCheetah/goldencheetah.github.io