docs: add keywords to Client.context

[ch3pjw]

The tutorial didn't work for me because Client.context() assumed the second argument was the port, not the username. Using keyword arguments solves the problem.

What do these changes do?

Make the tutorial work with aioftp == 0.18.0

Are there changes in behavior for the user?

Only that the tutorial should work out the box!

Related issue number

Checklist

• [x] I think the code is well written
• [ ] Unit tests for the changes exist
• [x] Documentation reflects the changes
• [ ] Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
  • .feature: Signifying a new feature.
  • .bugfix: Signifying a bug fix.
  • .doc: Signifying a documentation improvement.
  • .removal: Signifying a deprecation or removal of public API.
  • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[codecov[bot]]

Codecov Report

Merging #130 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #130   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files           6        6           
  Lines        1878     1878           
=======================================
  Hits         1860     1860           
  Misses         18       18           

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