doc: Fix the 30-minute intro to use += operator.

The 30-minute intro had a comment mentioning that `+=` will be included in the future. It's already included, and this fixes it to use `+=`.
auto merge of #16247 : TimDumol/rust/patch-1, r=alexcrichton

The 30-minute intro had a comment mentioning that `+=` will be included in the future. It's already included, and this fixes it to use `+=`.