[BUG] Env Var names are incorrect in docs

[jongio]

Here
https://github.com/Azure/azure-sdk-for-java/blob/master/sdk/identity/azure-identity/README.md#creating-a-service-principal-with-the-azure-cli

Has
AZURE_CLIENT_ID
AZURE_CLIENT_SECRET
AZURE_TENENT_ID

But, here does have underscores:
https://azuresdkdocs.blob.core.windows.net/$web/java/azure-identity/1.1.0-beta.1/index.html

Use the returned credentials above to set AZURECLIENTID(appId), AZURECLIENTSECRET(password) and AZURETENANTID(tenant) environment variables.