sync: incorrect exercise count when syncing introduction/instructions

Follow-up from https://github.com/exercism/problem-specifications/commit/5b6aedb722c28dbfe650f7b2a872bb9a4a922e48

### Current output

```console
$ bin/configlet sync --docs -u
Updating cached 'problem-specifications' data...
Checking exercises...
[warn] docs: instructions unsynced: gigasecond
[warn] docs: introduction missing: gigasecond
sync the above docs ([y]es/[n]o)? y
Updated the docs for 2 Practice Exercises
Every exercise has up-to-date docs!
```

### Expected output

The same, but either

> Updated the docs for 1 Practice Exercise

or something like:

> Updated 2 documentation files