Updating Sandbox API key procedure. (bunq/sdk_php#153)

[sandervdo]

//: # (Thanks for opening this pull request! Before you proceed please make sure that you have an issue that explains what this pull request will do. Make sure that all your commits link to this issue e.g. "My commit. (bunq/sdk_php#)". If this pull request is changing files that are located in "src/Model/Generated" then this pull request will be closed as these files must/can only be changed on bunq's side.)

This PR closes/fixes the following issues:

• Closes bunq/sdk_php#153
  • [x] Tested -> not necessary. Doc change.

[sandervdo]

@OGKevin updated.

[OGKevin]

@andrederoos