docs: update code example for custom TemplateSystem plugins
docs: update code example for custom TemplateSystem plugins

lucacerone/nikola,gwax/nikola,jjconti/nikola,okin/nikola,getnikola/nikola,knowsuchagency/nikola,wcmckee/nikola,getnikola/nikola,xuhdev/nikola,TyberiusPrime/nikola,gwax/nikola,schettino72/nikola,atiro/nikola,knowsuchagency/nikola,atiro/nikola,JohnTroony/nikola,okin/nikola,immanetize/nikola,atiro/nikola,schettino72/nikola,TyberiusPrime/nikola,berezovskyi/nikola,JohnTroony/nikola,JohnTroony/nikola,xuhdev/nikola,berezovskyi/nikola,okin/nikola,x1101/nikola,wcmckee/nikola,TyberiusPrime/nikola,masayuko/nikola,techdragon/nikola,masayuko/nikola,masayuko/nikola,knowsuchagency/nikola,s2hc-johan/nikola,lucacerone/nikola,pluser/nikola,techdragon/nikola,jjconti/nikola,wcmckee/nikola,okin/nikola,yamila-moreno/nikola,lucacerone/nikola,x1101/nikola,jjconti/nikola,techdragon/nikola,immanetize/nikola,xuhdev/nikola,getnikola/nikola,pluser/nikola,yamila-moreno/nikola,immanetize/nikola,andredias/nikola,s2hc-johan/nikola,gwax/nikola,getnikola/nikola,pluser/nikola,s2hc-johan/nikola,x1101/nikola,andredias/nikola,berezovskyi/nikola,yamila-moreno/nikola,andredias/nikola,schettino72/nikola,xuhdev/nikola