doc: move misplaced comma

Also:

* Remove unseeming repetition.
* By now, the reader has already heard that Rust is safe by default, so
reduce the overlong sentence, making it easier to read.
auto merge of #16717 : tshepang/rust/misplaced-comma, r=pcwalton

Also:

* Remove unseeming repetition.
* By now, the reader has already heard that Rust is safe by default, so
reduce the overlong sentence, making it easier to read.