Allow json output to work when using terraform viewcode

Before this patch, if you tried to produce JSON output from sphinx while using the sphinx_terraform.viewcode Sphinx extension, you would get:

File "/venv/lib/python3.10/json/encoder.py", line 182, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type PosixPath is not JSON serializable

This is because the title key in the context for the code page object in sphinx_terraform.viewcode.html.py:HtmlWriter.gen_sourcecode_html() was set to a pathlib.Path instead of a string.

This patch forces title to be a string.