docs/index.rst: Updated tutorial to native coroutines

[Pradhvan]

This updates the old-styled @asyncio.coroutine -> async def coroutine and updates yield from -> await.

Closes #436

What do these changes do?

This PR updates the old-styled @asyncio.coroutine -> async def coroutine and updates yield from -> await keyword.

Are there changes in behavior for the user?

Yes

Related issue number

None

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[codecov]

Codecov Report

Merging #437 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #437   +/-   ##
=======================================
  Coverage   92.28%   92.28%           
=======================================
  Files           9        9           
  Lines        1180     1180           
  Branches      172      172           
=======================================
  Hits         1089     1089           
  Misses         64       64           
  Partials       27       27

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[asvetlov]

Thanks!
Travis error is unrelated