Dumpcap manual: remove duplicate '-i' in synopsis

Gerald Combsrequested to merge
cherry-pick-f627f850 into master-3.2

Commit 3398c289 duplicated the '-i' option in the synopsis. Remove this again.

Change-Id: I85fb78515910b11e9dff9b3aa876746b2ff11fa4 Reviewed-on: https://code.wireshark.org/review/36678 Reviewed-by: Anders Broman a.broman58@gmail.com

(cherry picked from commit f627f850)

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