Update syntax in style.md

[lcpulzone]

An apostrophe was missing from one word and placed in the wrong spot for another word. This PR adds and moves an apostrophe.

[CLAassistant]

All committers have signed the CLA.

[mway]

Hey @lcpulzone, thanks for the PR!

The current state of the guide ("Dos and Don'ts", though "Do's and Don'ts" also appears to be acceptable) is aligned with guidelines and common use across different style guides and publications, as far as I can tell. Some examples:

• https://www.chicagomanualofstyle.org/qanda/data/faq/topics/Plurals/faq0008.html
• https://dictionary.cambridge.org/dictionary/english/dos-and-don-ts
• https://www.economist.com/whichmba/application-dos-and-donts
• https://www.nytimes.com/2009/05/27/dining/27cbox.html
• https://developer.apple.com/design/tips/
• https://www.theguardian.com/law/2017/nov/21/the-dos-and-donts-of-writing-a-personal-statement-for-law

I think this is OK to leave as-is. Even if there are counter arguments, the intent is clear; it's aligned enough that I don't think we're blatantly misspelling it.

Also, note that style.md should not be updated directly - it is generated, and the source for this particular text is in src/intro.md. See CONTRIBUTING.md for more information.