Fix grammar

Author: claui

{{ cookiecutter.pypi_package_name }}/{{ cookiecutter.python_package_name }}/errors.py

@@ -2,5 +2,5 @@
 {% if cookiecutter.include_executable == "y" %}
 
 class CliError(Exception):
-    """An user-facing error message."""
+    """A user-facing error message."""
 {% endif -%}