improve basic usage in pool document

[crvv]

This example has some issues:

1. with (yield from pool) as conn has been deprecated in Replace yield from with await... #283
2. The cursor is not closed in the example. It uses context manager for conn but not for cursor.
3. The indent of line 28 is wrong.

Besides, the basic usage of pool is in the README.rst of the project.
I don't think one more example is valuable.

[codecov]

Codecov Report

Merging #347 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #347   +/-   ##
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  Coverage   92.28%   92.28%           
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  Files           9        9           
  Lines        1180     1180           
  Branches      172      172           
=======================================
  Hits         1089     1089           
  Misses         64       64           
  Partials       27       27

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[jettify]

Sort of agree, but I think good idea to rework example, instead of deleting it, more examples the better.

[Nothing4You]

thanks for the PR.

the incorrect indent was already fixed in #418.
I plan to remove the yield syntax with #582, which does not only cover this pool example.
I've created #665 to track closing the cursor in the example properly.