message: Fix typo in backtrack.rs
diff: // regexes on small inputs because of its memory requirements.
  //
  // In particular, this is a *bounded* backtracking engine. It retains worst
  -// case linear time by keeping track of the states that is has visited (using a
  +// case linear time by keeping track of the states that it has visited (using a
  // bitmap). Namely, once a state is visited, it is never visited again. Since a
  // state is keyed by `(instruction index, input index)`, we have that its time
  // complexity is `O(mn)` (i.e., linear in the size of the search text).
  