Fix parameters in testing/gcs_test.py

Some of the parameters in the test for the Google Storage mock were
misspelled. This previously was not caught because that Python library
wasn't typed, but the latest version has types and mypy started
rejecting the misspelled arguments. Fix the spelling.

tests/testing/gcs_test.py

@@ -24,7 +24,7 @@ def test_mock(mock_gcs: MockStorageClient) -> None:
     credentials = google.auth.default()
     signing_credentials = impersonated_credentials.Credentials(
         source_credentials=credentials,
-        target_principle="some-service-account",
+        target_principal="some-service-account",
         target_scopes="https://www.googleapis.com/auth/devstorage.read_only",
         lifetime=2,
     )
@@ -62,7 +62,7 @@ def test_mock_files(mock_gcs_file: MockStorageClient) -> None:
     credentials = google.auth.default()
     signing_credentials = impersonated_credentials.Credentials(
         source_credentials=credentials,
-        target_principle="some-service-account",
+        target_principal="some-service-account",
         target_scopes="https://www.googleapis.com/auth/devstorage.read_only",
         lifetime=2,
     )