Change will to would to discuss what don't occur

As the event won't occur, a conditional seems more appropriate

src/vec/vec-into-iter.md

@@ -66,7 +66,7 @@ impl<T> IntoIterator for Vec<T> {
         let cap = self.cap;
         let len = self.len;
 
-        // Make sure not to drop Vec since that will free the buffer
+        // Make sure not to drop Vec since that would free the buffer
         mem::forget(self);
 
         unsafe {