mutability fixes

src/doc/trpl/structs.md

@@ -55,8 +55,8 @@ fn main() {

This will print `The point is at (5, 0)`.

Rust does not support mutability at the field level, so you cannot write
something like this:
Rust does not support field mutability at the language level, so you cannot
write something like this:

```rust,ignore
struct Point {
@@ -82,8 +82,8 @@ fn main() {

    point.x = 5;

    let point = point; // this new binding is immutable
    let point = point; // this new binding can’t change now

    point.y = 6; // this causes an error, because `point` is immutable!
    point.y = 6; // this causes an error
}
```