Update README.md (Typo "a Azure"→"an Azure")

[hyoshioka0128]

Purpose

• Typo "a Azure"→"an Azure"
• https://learn.microsoft.com/en-us/samples/azure-samples/openai-aca-lb/openai-aca-lb/
• https://github.com/Azure-Samples/openai-aca-lb/blob/main/README.md

• PingMSFTDocs

Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Documentation content changes
[ ] Other... Please describe:

How to Test

• Get the code

git clone [repo-address]
cd [repo-name]
git checkout [branch-name]
npm install

• Test the code

What to Check

Verify that the following are valid

• ...

Other Information