Enforce passing in an instance of the aiohttp resolver, not a class

[orf]

I'm sorry, with #893 I made a mistake: You need to pass in an instance of the resolver and not the class itself 😞

This fixes that.

[codecov]

Codecov Report

Merging #906 (8c0adce) into master (f269b8e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #906      +/-   ##
==========================================
- Coverage   89.30%   89.30%   -0.01%     
==========================================
  Files          42       42              
  Lines        4573     4572       -1     
==========================================
- Hits         4084     4083       -1     
  Misses        489      489              

Flag	Coverage Δ
unittests	89.30% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
tests/test_config.py	100.00% <ø> (ø)
aiobotocore/config.py	100.00% <100.00%> (ø)

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