Update templated README.md file

Author: nand4011

README.md

@@ -49,9 +49,7 @@ pub async fn main() -> Result<(), MomentoError> {
     let cache_client = CacheClient::builder()
         .default_ttl(Duration::from_secs(60))
         .configuration(Laptop::latest())
-        .credential_provider(CredentialProvider::from_env_var(
-            "MOMENTO_API_KEY".to_string(),
-        )?)
+        .credential_provider(CredentialProvider::from_default_env_var_v2()?)
         .build()?;
 
     cache_client.create_cache(CACHE_NAME.to_string()).await?;