Fix exit code text in bin/find-missing-links.sh

The code 5 was missing. The reason other lines changed is simply
formatting (aligned to 4 chars).

bin/find-missing-links.sh

@@ -138,6 +138,7 @@ Exit codes:
      2		-h and help string printed or -V and version string printed
      3		command line error
      4		bash version is too old
+     5		git not installed or not in \$PATH
      6		problems found with or in the topdir or topdir/YYYY directory
      7		problems found with or in the entry topdir/YYYY/dir directory
  >= 10		internal error