Fix license identifier

[limenet]

Currently the license identifier (Apache 2) does not comform to the SPDX list of valid identifiers at https://spdx.org/licenses/. Having a valid identifier helps automated license checking tools verify your package. The correct identifier is Apache-2.0

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@34189ff). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #13   +/-   ##
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  Coverage          ?   89.85%           
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  Files             ?        2           
  Lines             ?       69           
  Branches          ?        0           
=========================================
  Hits              ?       62           
  Misses            ?        7           
  Partials          ?        0

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