docs: correct word and space in SETUPTOOLS_SCM_PRETEND_VERSION warning

[antazoey]

• Replaces its with it is. The word its means something else; it's was definitely intended.
• Removes an extra space.

NOTE: I am debugging my project's version files are no longer being generated, both locally and in CI but I came across this doc issue so I thought I'd open the PR... But really, my priority here is to figure out why my version file doesn't get created anymore. I tried switching write_to to version_file but that did not seem to work.

I have been looking at #870 to try and find answers.

[antazoey]

Perhaps I should open a separate issue, but I am having to pin <8 on my project because the version file refuses to generate. I am guessing I am doing something wrong but I can't figure out what. I am going to keep debugging.

[RonnyPfannschmidt]

pleae create a own issue and addthe output when using SETUPTOOLS_SCM_DEBUG while building

[antazoey]

It sounds like my issue may be this one: #919

[RonnyPfannschmidt]

thanks for those fixes 👍