update readme for getByLabelText

[JonahMoses]

What: getByLabelText on readmy had username-input but should be username

Why: So docs are as accurate as they can be

How: readme pr

Checklist:

• Documentation N/A
• Tests N/A
• Ready to be merged
• Added myself to contributors table

[codecov]

Codecov Report

Merging #24 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #24   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines          53     53           
  Branches       13     13           
=====================================
  Hits           53     53

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[kentcdodds]

Perfect!