Update build_meta.rst
Update build_meta.rst

Mainly grammar/style tweaks... which turned into a few more thoughts:
About this statement: "decide which program to use to 'build from source' (the default is ``setuptools``)." 
I think 'default' conveys the wrong impression here when it is explaining the new way of doing things. PEP517 says, "(build) tools should revert to the legacy behaviour of running setup.py" Maybe a footnote would be better saying it is a fallback, for backward compatibility. (But, all the same, this is interesting information that now explains to me why 'py -m build' worked before I had tried using a pyproject.toml file.)
Line 37:  ``build_meta`` implements ``setuptools``'s build system support.
This seems out of place, and doesn't make sense. build_meta hasn't been mentioned yet in this article. Maybe this sentence could go before line 61.
Line 26-28: This seems like low-level detail that is out of place in this article. If you want to keep it, maybe add a note that the default directory is dist\.
**The "bullet items" under the PEP517 section aren't formatting correctly in the Preview. Please check that I haven't messed anything up there in the final render.
pypa/setuptools,pypa/setuptools,pypa/setuptools