Fix bug with API.sensors.async_get_sensor not resolving

[bachya]

Describe what the PR does:

We were looking at the wrong endpoint (/v1/sensor/:sensor_index when it should be /v1/sensors/:sensor_index).

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

[codeclimate]

Code Climate has analyzed commit ee5ba65 and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #23 (ee5ba65) into dev (16a7479) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               dev       #23   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines          481       481           
=========================================
  Hits           481       481           

Impacted Files	Coverage Δ
aiopurpleair/endpoints/sensors.py	100.00% <ø> (ø)
aiopurpleair/models/sensors.py	100.00% <ø> (ø)

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