Slice used before it is introduced

[sahi1l]

Describe the bug Section 6.5 includes sequences-5-3 under Check Your Understanding which uses a slice; however, slices aren't introduced until the next section.

https://runestone.academy/runestone/static/fopp/Sequences/Length.html

[bnmnetp]

Yep, this question should just be moved to the next section.

[dichharai]

PR #188 has this change.