Fix placeholder typo

[PascalLike]

Overview

I'm using vector tiles with pg_tileserv, but I get this error:

  File "/pygeoapi/pygeoapi/provider/mvt.py", line 222, in get_tiles
    resp = session.get(f'{base_url}/{layer}/{z}/{y}/{x}.{f}{url_query}')  # noqa
NameError: name 'f' is not defined

It looks like a typo in the variable name for the URL placeholder, called f (variable not defined anywhere else) instead of format_, the extension for the tile.

Related Issue / Discussion

I didn't create an issue. It's a very simple error 😇

Additional Information

Contributions and Licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution.
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines