docs(async_client): Fix typo in get_mtls_endpoint_and_cert_source doc (#662)

Co-authored-by: Mariatta Wijaya <Mariatta@users.noreply.github.com>

Author: Bob "Wombat" Hogg

packages/google-cloud-firestore/google/cloud/firestore_admin_v1/services/firestore_admin/async_client.py

@@ -185,7 +185,7 @@ def get_mtls_endpoint_and_cert_source(
         The API endpoint is determined in the following order:
         (1) if `client_options.api_endpoint` if provided, use the provided one.
         (2) if `GOOGLE_API_USE_CLIENT_CERTIFICATE` environment variable is "always", use the
-        default mTLS endpoint; if the environment variabel is "never", use the default API
+        default mTLS endpoint; if the environment variable is "never", use the default API
         endpoint; otherwise if client cert source exists, use the default mTLS endpoint, otherwise
         use the default API endpoint.