Rollup merge of rust-lang#36711 - pcn:patch-1, r=steveklabnik

Suggesting a change to a comment that puzzled me

While reading this, the comment made it difficult for me to simply absorb the concept.  It interrupted my reading flow, and I think this expresses the same meaning, but reads a bit better.  It's trivial, but makes it easier for me to move on to the next line.

Author: steveklabnik

src/doc/book/references-and-borrowing.md

@@ -86,7 +86,7 @@ fn main() {
         return v.iter().fold(0, |a, &b| a + b);
     }
     // Borrow two vectors and sum them.
-    // This kind of borrowing does not allow mutation to the borrowed.
+    // This kind of borrowing does not allow mutation through the borrowed reference.
     fn foo(v1: &Vec<i32>, v2: &Vec<i32>) -> i32 {
         // do stuff with v1 and v2
         let s1 = sum_vec(v1);