docs: modified examples using the wrong keyword (yield -> await) (#1021)

docs/api/request.md

@@ -167,7 +167,7 @@ if (ctx.fresh) {
 
 // cache is stale
 // fetch new data
-ctx.body = yield db.find('something');
+ctx.body = await db.find('something');
 ```
 
 ### request.stale

docs/api/response.md

@@ -234,7 +234,7 @@ ctx.type = 'png';
 const minify = require('html-minifier');
 
 app.use(function * minifyHTML(next) {
-  yield next;
+  await next;
 
   if (!ctx.response.is('html')) return;