Reduce bundle size limit to reflect recent improvements.

Part of #2743.

The current bundle size is about 9.9KB, so 10KB gives us a tiny bit of
breathing room. Any PRs that push the react-apollo bundle over 10KB
deserve a closer look to make sure they're as conservative as they can be
(within reason).

package.json

@@ -54,7 +54,7 @@
   "bundlesize": [
     {
       "path": "./dist/bundlesize.js",
-      "maxSize": "11.5 KB"
+      "maxSize": "10 KB"
     }
   ],
   "jest": {