Fix Liquid warning in the single layout from the template

[oleg-nenashev]

IMHO it is a Liquid bug but easier to just get rid of it in our code

C:/Ruby27-x64/lib/ruby/gems/2.7.0/gems/json-1.8.6/lib/json/common.rb:155: warning: Using the last argument as keyword parameters is deprecated
    Liquid Warning: Liquid syntax error (line 1): Expected dotdot but found id in "(page.header.overlay_color or page.header.overlay_image) or page.header.image" in C:/Users/Oleg/Documents/opensource/wiremock/wiremock.org/_layouts/splash.html
    Liquid Warning: Liquid syntax error (line 1): Expected dotdot but found id in "(page.header.overlay_color or page.header.overlay_image) or page.header.image" in C:/Users/Oleg/Documents/opensource/wiremock/wiremock.org/_layouts/single.html
    Liquid Warning: Liquid syntax error (line 1): Expected dotdot but found id in "(page.header.overlay_color or page.header.overlay_image) or page.header.image" in C:/Users/Oleg/Documents/opensource/wiremock/wiremock.org/_layouts/archive.html
    Liquid Warning: Liquid syntax error (line 1): Expected dotdot but found id in "(page.header.overlay_color or page.header.overlay_image) or page.header.image" in C:/Users/Oleg/Documents/opensource/wiremock/wiremock.org/_layouts/support.html

mmistakes/minimal-mistakes#479. Contributes to #58