Update sentence in 'Plotting' lesson to improve tone & clarity

[sadielbartholomew]

Closes #705, plus add several extra words on the updated sentence to clarify the arguments to use to plt.plot which in my opinion is useful in addition and otherwise only implied i.e. to be picked up by assumption from the example code snippet - strictly a separate issue so added as a separate commit, but might as well raise in this PR (can revert if controversial but doubt it will be so!).

Please briefly summarise the changes made in the pull request, and the reason(s) for making these changes.

Update the episode/lesson 'Plotting' to avoid use of the subjective term 'simple', by replacing with the more specific and objective assertion on conciseness, with (whilst touching that sentence) a very small amount of text added to clarify what arguments plt.plot takes.

If any relevant discussions have taken place elsewhere, please provide links to these.

n/a

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[vahtras]

How about "just one call" -> "a single call"?

[sadielbartholomew]

Good point - if 'just' is indeed a problematic word it is somewhat ironic, though unintentional, I ended up suggesting it in wording to replace use of 'simple'! Will update in a new commit in a moment.

[sadielbartholomew]

Resolved in 1d882a2, though I'll let you mark this as formally resolved if you agree.