Cleaned up the developer guide.

[dblock]

Description

Took me some time to figure out how to run tests.
A small cleanup of the dev guide.

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[codecov]

Codecov Report

Merging #388 (a1052dd) into main (2abb8c4) will increase coverage by 0.59%.
The diff coverage is 100.00%.

❗ Current head a1052dd differs from pull request most recent head 87f35d8. Consider uploading reports for the commit 87f35d8 to get more accurate results

@@            Coverage Diff             @@
##             main     #388      +/-   ##
==========================================
+ Coverage   71.44%   72.04%   +0.59%     
==========================================
  Files          77       77              
  Lines        7190     7190              
==========================================
+ Hits         5137     5180      +43     
+ Misses       2053     2010      -43     

Impacted Files	Coverage Δ
opensearchpy/_async/http_aiohttp.py	82.81% <100.00%> (+3.12%)	⬆️

... and 16 files with indirect coverage changes

[VachaShah]

The macos tests seem to be failing overall in the repo for TestConnectionHttpbin.test_urllib3_connection, @dblock any insights?

[VachaShah]

LGTM!

[saimedhi]

The macos tests seem to be failing overall in the repo for TestConnectionHttpbin.test_urllib3_connection, @dblock any insights?

Httpbin is replaced with Http server in this PR. So, now tests will not fail if this branch is synced with main.