docs(CellMeasurer): fix import statement (bvaughn#1187)

The `Grid` component isn't used in this example, it was probably a remnant
of copy-pasting or something.

Author: skipjack

docs/CellMeasurer.md

@@ -116,7 +116,7 @@ To support this, a function-child is passed to `CellMeasurer` which then receive
 
 ```jsx
 import React from 'react';
-import { CellMeasurer, CellMeasurerCache, Grid } from 'react-virtualized';
+import { CellMeasurer, CellMeasurerCache, List } from 'react-virtualized';
 
 // In this example, average cell height is assumed to be about 50px.
 // This value will be used for the initial `Grid` layout.