Fixed typo in condition exposure section of README
Fixed typo in condition exposure section of README

`if` should be `if:`
marshall-lee/grape-entity,ruby-grape/grape-entity,tyre/grape-entity,danhodge/grape-entity,calfzhou/grape-entity,dblock/grape-entity,etehtsea/grape-entity,geniousli/grape-entity,cakejelly/grape-entity,LeFnord/grape-entity,dan-corneanu/grape-entity,intridea/grape-entity,iangreenleaf/grape-entity