Rollup merge of #25257 - dpetersen:mutability-docfix, r=steveklabnik

I think there's a trivial missing word in the Mutability document. I reformatted the resulting paragraph in vim, which seems to match what the rest of the document is doing as far as word wrapping.

Edit: I found another minor thing as I continued reading.

P.S. I'm re-reading the docs, since so much has changed since my first read, and they've gotten even better! Nice job!

r? @steveklabnik