New example of `get_object`

[grigoriev-semyon]

Description of Change

I've added a description of how to use the get_object method when using AsyncExitStack.

Fixes: #1061

Checklist for All Submissions

• [x] I have added change info to CHANGES.rst (i dont think this is required when update exmaples)
• [x] If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
  • [x] Detailed description of issue
  • [x] Alternative methods considered (if any)
  • [x] How issue is being resolved
  • [x] How issue can be reproduced
• [x] If this is providing a new feature (can be provided via link to issue with these details):
  • [x] Detailed description of new feature
  • [x] Why needed
  • [x] Alternatives methods considered (if any)

[grigoriev-semyon]

thx to @SirGrigory for help in finding a solution to the problem

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (ab08687) 86.46% compared to head (861a6d0) 86.46%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1062 +/- ## ======================================= Coverage 86.46% 86.46% ======================================= Files 60 60 Lines 5814 5814 ======================================= Hits 5027 5027 Misses 787 787 ``` | [Flag](https://app.codecov.io/gh/aio-libs/aiobotocore/pull/1062/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/aio-libs/aiobotocore/pull/1062/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `86.46% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs#carryforward-flags-in-the-pull-request-comment) to find out more.

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[thehesiod]

ty!