Fix grammar in get-vpc-attachment example

[adev-code]

Changes:

• Corrected "To get an a VPC attachment" to "To get a VPC attachment" in https://docs.aws.amazon.com/cli/latest/reference/networkmanager/get-vpc-attachment.html

File modified:

• awscli/examples/networkmanager/get-vpc-attachment.rst

From a customer feedback: V2071738585 (Internal Tracking Only)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.38%. Comparing base (c4681b5) to head (5cdbb35).
⚠️ Report is 43 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9986      +/-   ##
===========================================
- Coverage    93.39%   93.38%   -0.01%     
===========================================
  Files          210      210              
  Lines        17052    17055       +3     
===========================================
+ Hits         15925    15927       +2     
- Misses        1127     1128       +1     

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[adev-code]

Testing:

[kdaily]

Looks good to me! Let's run the cherry pick GitHub Action to port this to v2 and then I can approve that one too, then both can be merged.