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?

src/doc/tutorial.md

@@ -1395,7 +1395,7 @@ let mut y = ~5; // mutable
 In contrast with
 owned boxes, where the holder of an owned box is the owner of the pointed-to
 memory, references never imply ownership - they are "borrowed".
-A reference can be borrowed to
+You can borrow a reference to
 any object, and the compiler verifies that it cannot outlive the lifetime of
 the object.