Off-by-one in check-syntax

[sorawee]

Consider:

#lang rhombus

fun
| factorial(0): 1
| factorial(n): n*factorial(n-1)

When hovering on *, n is highlighted.

Note that if we add spaces around *:

n * factorial(n-1)

then hovering on * no longer highlights n.

This seems to indicate that there's an off-by-one bug somewhere.