Fix gramatical error in validation message.  Fixes #511
Fix gramatical error in validation message.  Fixes #511

niebert/json-editor,flibbertigibbet/json-editor,LACabeza/json-editor,juazugas/json-editor,JJediny/json-editor,abe1001/json-editor,ZECTBynmo/json-editor,abop/json-editor,JJediny/json-editor,jdorn/json-editor,azavea/json-editor,abop/json-editor,niebert/json-editor,rmulder/json-editor,grokify/json-editor,cunneen/json-editor,grokify/json-editor,YousefED/json-editor,cunneen/json-editor,YousefED/json-editor,LACabeza/json-editor,azavea/json-editor,juazugas/json-editor,JJediny/json-editor,jdorn/json-editor,abe1001/json-editor,rmulder/json-editor,ZECTBynmo/json-editor,flibbertigibbet/json-editor