Fix: correct notation for longitudinal arg value

[octomike]

fix a trivial issue when setting the longitudinal argument:

smriprep --longitudinal /in /out participant

The "--longitudinal" flag is deprecated. Use "--subject-anatomical-reference unbiased" instead.
Traceback (most recent call last):
  File "/home/mk/.local/bin/smriprep", line 7, in <module>
    sys.exit(main())
             ~~~~^^
  File "/home/mk/work/src/smriprep/src/smriprep/cli/run.py", line 37, in main
    opts['longitudinal'] = True
    ~~~~^^^^^^^^^^^^^^^^
TypeError: 'Namespace' object does not support item assignment

We have to use the dot notation to change argparse arg's namespace attributes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.63%. Comparing base (bcb3a73) to head (b99232d).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #508   +/-   ##
=======================================
  Coverage   70.63%   70.63%           
=======================================
  Files          25       25           
  Lines        2077     2077           
  Branches      270      270           
=======================================
  Hits         1467     1467           
  Misses        552      552           
  Partials       58       58           

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