Do not use `[roots]` in example `[tokens]`

Use of `[roots]` in `[tokens]` is tricky, because final value of a
`[roots:foo]` will "silently" depend on DESTDIR, while DESTDIR is
only known during install time.  On the other hand, `[tokens]` are used only
during build time.

In other words, YOU CANNOT RELY on the value of keys in `[roots]` to be
consistent.  For example, consider root:

    [roots]
        foo = /bar
        qux = [ENV:PREFIX]:/quux

`[roots:foo]` will be '/bar' at build time, but "$DESTDIR/bar" at
install time, and `[roots:qux]` will be "$PREFIX/quux" at build time,
but "$DESTDIR/$PREFIX/quux" at install time.

Just about only case when you might want to use it is mentioning it in
a document, when you want the value to be rather "standard" than
"correct", while you still want to respect PREFIX.

So let's not lead new users directly into that rabbit hole.

src/stub

                 echo "    doc     = [ENV:PREFIX]/share/doc/$PackageName"
                 echo ""
                 echo "[tokens]"
-                echo "    __BIN_DIR__ = [roots:bin]"
+                echo "    __${PackageName^^}_FOO__ = Barr.."
                 echo ""
                 echo "[modes]"
                 echo "    bin     = 755"