tree-wide: fix spelling of "fallback"

Similarly to "setup" vs. "set up", "fallback" is a noun, and "fall back"
is the verb. (This is pretty clear when we construct a sentence in the
present continous: "we are falling back" not "we are fallbacking").

src/kernel-install/90-loaderentry.install

@@ -95,7 +95,7 @@ for initrd in "${INITRD_OPTIONS[@]}"; do
     fi
 done
 
-# If no initrd option is supplied, fallback to "initrd" which is
+# If no initrd option is supplied, fall back to "initrd" which is
 # the name used by dracut when generating it in its kernel-install hook
 [[ ${#INITRD_OPTIONS[@]} == 0 ]] && INITRD_OPTIONS=( initrd )