Use ngettext to handle plurality

[victorlin]

Follow-up to #1591 (comment)

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

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Project coverage is 71.14%. Comparing base (1e9d131) to head (b06ac37).
Report is 58 commits behind head on master.

Files with missing lines	Patch %	Lines
augur/filter/_run.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1596      +/-   ##
==========================================
+ Coverage   71.02%   71.14%   +0.11%     
==========================================
  Files          79       79              
  Lines        8256     8273      +17     
  Branches     2003     2010       +7     
==========================================
+ Hits         5864     5886      +22     
+ Misses       2101     2099       -2     
+ Partials      291      288       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.