Update the "Additional tutorials" link
str: Inline `only_ascii` in string iterators.

Was killing performance of selector matching in Servo.
auto merge of #13673 : bmac/rust/utorial-fix, r=alexcrichton

This pr updates the "Additional tutorials" link in the scope section to match the "What next?" link in the table of contents.
auto merge of #13674 : pcwalton/rust/more-str-inlines, r=alexcrichton

Was killing performance of selector matching in Servo.

r? @alexcrichton (or anyone)