minor suggestions for flox-vs-containers.md

[robinbrantley]

mostly grammar and formatting suggestions.

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[robinbrantley]

@zmitchell I am also thinking the last section re sending to production is a bit weak and perhaps the Flox bit could be strengthened or clarified a bit by stating what flox containerize produces instead of only what it doesn't produce?

[zmitchell]

Addressed in https://github.com/flox/floxdocs/pull/78