doc: fix :zephyr-app: paths

During testing I've found one bad path and looked if
there is more.

I've used this oneliner to find more:

```
rg :zephyr-app: | awk '{ print $3 }' | while read dir
do
    test -d $dir || echo $dir
done | grep '^samples' | grep -v '<' | sort | uniq

```

Signed-off-by: Nerijus Bendžiūnas <nerijus.bendziunas@gmail.com>