fixing a bug for checking if to use Azure Open AI (#277)

## Purpose
<!-- Describe the intention of the changes being proposed. What problem
does it solve or functionality does it add? -->
* ...

## Does this introduce a breaking change?
<!-- Mark one with an "x". -->
```
[ ] Yes
[ ] No
```

## Pull Request Type
What kind of change does this Pull Request introduce?

<!-- Please check the one that applies to this PR using "x". -->
```
[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] 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
<!-- Add steps to run the tests suite and/or manually test -->
```
```

## What to Check
Verify that the following are valid
* ...

## Other Information
<!-- Add any other helpful information that may be needed here. -->

---------

Co-authored-by: Xiaoyun Zhang <[email protected]>

Author: CawaMS

app/prepdocs/PrepareDocs/Program.Clients.cs

@@ -166,10 +166,11 @@ private static Task<OpenAIClient> GetOpenAIClientAsync(AppOptions options) =>
        {
            if (s_openAIClient is null)
            {
-               var useAOAI = Environment.GetEnvironmentVariable("UseAOAI") == "true";
+               var useAOAI = Environment.GetEnvironmentVariable("USE_AOAI") == "true";
                if (!useAOAI)
                {
                      var openAIApiKey = Environment.GetEnvironmentVariable("OPENAI_API_KEY");
+                     Console.WriteLine("useAOAI value is: " + useAOAI.ToString());
                      ArgumentNullException.ThrowIfNullOrEmpty(openAIApiKey);
                      s_openAIClient = new OpenAIClient(openAIApiKey);
                }