fix link to mustache spec in helpers.md
fix link to mustache spec in helpers.md

ThesaurusSoftware/GRMustache,lumoslabs/GRMustache,mz2/GRMustache,sdonly/GRMustache,lumoslabs/GRMustache,sdonly/GRMustache,ipmobiletech/GRMustache,samdeane/GRMustache,groue/GRMustache,lumoslabs/GRMustache,samdeane/GRMustache,saim80/GRMustache,ipmobiletech/GRMustache,samdeane/GRMustache,ThesaurusSoftware/GRMustache,ipmobiletech/GRMustache,sdonly/GRMustache,mz2/GRMustache,saim80/GRMustache,gamechanger/GRMustache,mz2/GRMustache,ThesaurusSoftware/GRMustache,gamechanger/GRMustache,saim80/GRMustache,groue/GRMustache,groue/GRMustache