link to `Set up an AWS account with admin permissions` is broken

[Jannes Stubbemann]

In

# Usage
## Setup
| **Step** | **Command** |**Description**|
|---|-------|------|
|  1. | `npm install -g serverless` | Install Serverless CLI  |
|  2. | `npm install` | Install Serverless dependencies  |
|  3. | Set up an AWS account with admin permissions | [Documentation](https://github.com/serverless/serverless/blob/master/docs/providers/aws/guide/credentials.md)  |

The link for Set up an AWS account with admin permissions returns a 404. From the serverless repo it looks like this is the new link.

[Benny Bauer]

Thanks! I've updated it now (8c76d75)