tckglobal: Revert documentation regression
tckglobal: Revert documentation regression

The tckglobal example usage was modified in 47b9fec8 to suggest 1e9 iterations. However this was somehow reverted in 7cd8940c. The change was picked up due to a mismatch in the automatically-generated documentation by Sphinx within TravisCI test 2462.2.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3