Fix license in citation file

[sethfischer]

Incorrect license value caused a parsing error.

Syntax of CITATION.cff was checked with cffconvert which identified the value for license should be "Apache-2.0" rather than "Apache 2.0".

$ cffconvert --validate
jsonschema.exceptions.ValidationError: 'Apache 2.0' is not one of […, 'Apache-1.0', 'Apache-1.1', 'Apache-2.0',…
⋮

Closes #1886

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.74%. Comparing base (dadf151) to head (e0c344c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1916   +/-   ##
=======================================
  Coverage   95.74%   95.74%           
=======================================
  Files          29       29           
  Lines        7827     7827           
  Branches     1179     1179           
=======================================
  Hits         7494     7494           
  Misses        192      192           
  Partials      141      141           

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[jmwright]

Confirmed, thanks @sethfischer