Fix curate --help

[joverlee521]

Description of proposed changes

Limit the format-dates help to the first line, which does not include the %Y bit that was causing the error.

Related issue(s)

Resolves #1753

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[victorlin]

+1 for using first_line here, not because it avoids %Y (not a good enough reason IMO), but because it keeps the output of augur curate -h tidy.

A broader suggestion would be to tidy the rest of that help text. There are some existing usages of first_line where it is cut off mid-sentence, e.g.

    abbreviate-authors  Abbreviates a full list of authors to be '<first author> et al.' of the NDJSON
    parse-genbank-location
                        Parses GenBank's 'location' field of the NDJSON record to 3 separate
    transform-strain-name
                        Verifies strain name pattern in the 'strain' field of the NDJSON

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.12%. Comparing base (e6df775) to head (1d6724e).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
augur/curate/titlecase.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1755   +/-   ##
=======================================
  Coverage   73.11%   73.12%           
=======================================
  Files          79       79           
  Lines        8350     8353    +3     
  Branches     1704     1704           
=======================================
+ Hits         6105     6108    +3     
  Misses       1958     1958           
  Partials      287      287           

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