docs: add --prefix=/usr to Arch build instructions

Without it meson defaults to /usr/local, so the installed paths don't
match the README and DEFAULT_PATH compiled into the PAM module is wrong.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

readme.md

@@ -49,10 +49,13 @@ Then follow the [Wayland](#usage-linux---wayland-only-gtklock) or [X11](#usage-l
 
 ```bash
 sudo pacman -S meson ninja rust gtk3 gstreamer gst-plugins-base gst-plugins-good gst-plugins-bad gtklock
-meson setup builddir && meson compile -C builddir
+meson setup builddir --prefix=/usr
+meson compile -C builddir
 sudo meson install -C builddir
 ```
 
+`--prefix=/usr` matches Arch conventions (same as pacman) and ensures the compiled-in default path for `ahfail-display` matches where it is installed.
+
 Installs:
 - `/usr/lib/gtklock/ahfail-module.so` — gtklock module
 - `/usr/lib/ahfail/libahfail_pam.so` — PAM module (for X11)