Update liquid docs for named parameters

[graygilmore]

The YARD liquid gem now supports specifying named parameters. For the core liquid tags and filters this is the only object I could find that needed to be updated.

To specify a named parameter you need to add a : to the end of the name:

# @liquid_optional_param foo [string] This is a positional parameter
# @liquid_optional_param bar: [string] This is a named parameter

Adding a colon will make sure that the newly added positional attribute in the JSON docs will be set to false.