Fixed react key warning for loki start page

public/app/plugins/datasource/loki/components/LokiCheatSheet.tsx

@@ -26,7 +26,7 @@ export default (props: any) => (
   <div>
     <h2>Loki Cheat Sheet</h2>
     {CHEAT_SHEET_ITEMS.map(item => (
-      <div className="cheat-sheet-item" key={item.expression}>
+      <div className="cheat-sheet-item" key={item.title}>
         <div className="cheat-sheet-item__title">{item.title}</div>
         {item.expression && (
           <div