[Typo] Exercise 2 in chapter 9 should be plusR_succ_left instead of plus_succ_left?

[spearman]

fp-lean/functional-programming-lean/src/tactics-induction-proofs.md

Line 283 in 7eefe73

* Rewrite the proof of `plus_succ_left` to use `<;>` in a single line.

The exercise states:

• Rewrite the proof of plus_succ_left to use <;> in a single line.

plus_succ_left doesn't appear anywhere else in the text, but plusR_succ_left does.

[david-christiansen]

Thanks for the report! I'm knee-deep in work on the Lean reference manual, but an update to FPiL is planned for after that. I'll fix it then, if not before.