Fix webpack command in tutorials (#1471)

Author: AndiDog

net/grpc/gateway/examples/echo/tutorial.md

@@ -191,7 +191,7 @@ into one single JS library that can be used in the browser.
 
 ```sh
 $ npm install
-$ npx webpack client.js
+$ npx webpack ./client.js
 ```
 
 Now embed `dist/main.js` into your project and see it in action!

net/grpc/gateway/examples/helloworld/README.md

@@ -284,7 +284,7 @@ can be consumed by the browser.
 
 ```sh
 $ npm install
-$ npx webpack client.js
+$ npx webpack ./client.js
 ```
 
 Here we use `webpack` and give it an entry point `client.js`. You can also use