Allow for white space in kernelspec regex

Before this fix, updating the kernelspec would fail if there was no
whitespace after the colon following "kernelspec", e.g. if the JSON was
minified

jupyter-images/shared/default_kernel.py

@@ -41,7 +41,7 @@ def update_kernelspec_in_notebooks(directory, new_name):
             # (including nested structures), preserving the overall structure
             # and formatting of the file.
             updated_contents = re.sub(
-                r'"kernelspec": \{.*?\}',
+                r'"kernelspec": *\{.*?\}',
                 f'"kernelspec": {updated_kernelspec}',
                 file_contents, flags=re.DOTALL
             )