userguide: Un-hide a TODO block completed in 2011

The userguide contained a commented-out section for marks, which
included the line:

> TODO: make i3-input replace %s

The line was added in a26a11c6099f82fc9fd8b87f8bda128408b4f7c9 (May
2011), at which point i3-input did not have a -F switch. The switch
was added only in 1737a78fcd8025e11398dbcf5acd65c6a07ae86d (September
2011), but the documentation was never updated to enable the
commented-out examples.