Readme typo

### Description of defect

The Readme says, "... to the Azure IoT Hub service". However this repo is for the AWS IoT core service. This is a simple bug report, yet the bot has requested that I pad out the description to 25 words. So there you go.

#### Target(s) affected by this defect ?
n/a

#### Toolchain(s) (name and version) displaying this defect ?
n/a

#### What version of Mbed-os are you using (tag or sha) ?
n/a

#### What version(s) of tools are you using. List all that apply (E.g. mbed-cli)
n/a

#### How is this defect reproduced ?
n/a

Open the repository README and find the sentence that refers to the Azure IoT Hub service. Update that reference to AWS IoT Core, then review the rendered README to confirm the wording is correct.

Written by the indexing model from the issue text.