correct a typo in a filter string

svn path=/trunk/; revision=54687

Change-Id: Ie9ca9fbffdfe83b74f38ab314ba4403d2f17ee8e
Reviewed-on: https://code.wireshark.org/review/104
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>