Fix return type for setup_aiohttp_apispec

[KurganskyVladimir]

Now setup_aiohttp_apispec returns AiohttpApiSpec instance and we need to fix return type.

[codecov-commenter]

Codecov Report

Merging #124 (b78c894) into master (34ccc58) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #124   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files           8        8           
  Lines         242      242           
=======================================
  Hits          231      231           
  Misses         11       11           

Impacted Files	Coverage Δ
aiohttp_apispec/aiohttp_apispec.py	95.07% <ø> (ø)

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