Copy-edit a sentence about borrowing references
auto merge of #13062 : mbrubeck/rust/doc-edit, r=alexcrichton

This is a very minor edit to the tutorial section on references.

Reading this section for the first time, I stumbled on the phrase "a reference can be borrowed to any object."  Its meaning was clear enough once I got it, but I had to re-read it a couple of times to parse it correctly.  Something about the passive voice plus the way "reference to any object" is split up by the verb phrase.  How about this instead?