cmd-buildinitramfs-fast: fix example install invocation
It uses incorrect syntax for creating leading directories, and doesn't
place Ignition in /usr/bin.
@@ -10,7 +10,7 @@ set -euo pipefail
# ```
# $ cd /src/ignition
# $ make
-# $ install -D -d -m 0755 bin/amd64/ignition /srv/fcos/overrides/initramfs
+# $ install -D -m 0755 -t /srv/fcos/overrides/initramfs/usr/bin bin/amd64/ignition
# $ cd /srv/fcos
# $ cosa buildinitramfs-fast
# ```
