[doc] fix typo of AWS Secrets Manager

[t-kikuc]

What this PR does / why we need it:

Replaced AWS SecretManager with AWS Secrets Manager

Which issue(s) this PR fixes: no

Does this PR introduce a user-facing change?: no

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[ffjlabo]

Nice Catch 👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ea9a453) 31.02% compared to head (eab2f31) 31.03%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4770      +/-   ##
==========================================
+ Coverage   31.02%   31.03%   +0.01%     
==========================================
  Files         225      225              
  Lines       26353    26353              
==========================================
+ Hits         8175     8179       +4     
+ Misses      17525    17521       -4     
  Partials      653      653              

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