revise seqtk boolean parameter example not to use shift quality

[peterjc]

Currently docs/_writing_parameters.rst uses this as a boolean parameter example:

-V        shift quality by '(-Q) - 33'

This seems inappropriate and rather than exposing it as a user setting it should be automatic conditional on the datatype (i.e. use this for fastqillumina only). This could be used in a more complex example using conditionals in the Cheetah command tag and multiple input types.

I would suggest we replace the boolean parameter -V example with something simpler like -C or -N:

-C        drop comments at the header lines
-N        drop sequences containing ambiguous bases

[hexylena]

xref https://github.com/galaxyproject/tools-iuc/issues/204

[bgruening]

ping @martenson Fix was here: https://github.com/galaxyproject/tools-iuc/commit/f987b452b29c8b13181c7093c077e2612dd1ac1b