docs: update examples to use stable key prop

Author: Clauderic Demers

examples/basic.js

@@ -26,7 +26,7 @@ class App extends Component {
     return (
       <SortableContainer onSortEnd={this.onSortEnd}>
         {items.map((value, index) => (
-          <SortableItem key={`item-${index}`} index={index} value={value} />
+          <SortableItem key={`item-${value}`} index={index} value={value} />
         ))}
       </SortableContainer>
     );

examples/drag-handle.js

@@ -37,7 +37,7 @@ class App extends Component {
     return (
       <SortableContainer onSortEnd={this.onSortEnd} useDragHandle>
         {items.map((value, index) => (
-          <SortableItem key={`item-${index}`} index={index} value={value} />
+          <SortableItem key={`item-${value}`} index={index} value={value} />
         ))}
       </SortableContainer>
     );

examples/react-infinite.js

@@ -16,7 +16,7 @@ const SortableInfiniteList = sortableContainer(({items}) => {
     >
       {items.map(({value, height}, index) => (
         <SortableItem
-          key={`item-${index}`}
+          key={`item-${value}`}
           index={index}
           value={value}
           height={height}