correct a typo in a filter string

svn path=/trunk/; revision=54687

Change-Id: I15832d91824087a2fed50a3968cb2801cc30cf91
Reviewed-on: https://code.wireshark.org/review/105
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>