Minor improvement in man page for --server-max-duration flag

[manedurphy]

PLEASE NOTE the following text from the iperf3 license. Submitting a
pull request to the iperf3 repository constitutes "[making]
Enhancements available...publicly":

You are under no obligation whatsoever to provide any bug fixes, patches, or
upgrades to the features, functionality or performance of the source code
("Enhancements") to anyone; however, if you choose to make your Enhancements
available either publicly, or directly to Lawrence Berkeley National
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The complete iperf3 license is available in the LICENSE file in the
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• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies: master

• Issues fixed (if any): N/A

• Brief description of code changes (suitable for use as a commit message):

  • Small improvement to man page to show that the --server-max-duration flag requires an argument

[bmah888]

Thanks! Merged.