Fix manpage warnings in mp4box.1
Fix manpage warnings in mp4box.1

Running `LC_ALL=en_US.UTF-8 MANROFFSEQ='' MANWIDTH=80 man --warnings -E UTF-8 -l -Tutf8 -Z mp4box.1 >/dev/null` to check the manpage syntax will otherwise return the following warnings:
<standard input>:396: warning: numeric expression expected (got `f')
<standard input>:623: warning: numeric expression expected (got `O')

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