Dahsboard strings should use numbered print placeholders

[nbachiyski]

Two strings in wp-admin/inncludes/dashboard.php are using consecutive %s instead of %1$s and %2$s.

[nacin]

There's no need to have numbered placeholders here, as it's impossible for the href to come after the anchor. See my commit message for [18297].

[nbachiyski]

Yes, makes sense. I find that backslashes not very pretty, either.