Fix REST API sample URL as it appears samples are consolidated to the nanoFramework repo

[cammurray]

Description

• README.MD has a broken link to the REST sample. This link is no longer valid and it appears that nanoFramework has consolidated all the samples in to a big samples folderon the main repo.

Motivation and Context

• Fixes up the documentation so that it is not referring to a broken link.

How Has This Been Tested?

• Previewed this README.MD file and it links correctly after this PR.

Screenshots

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Config and build (change in the configuration and build system, has no impact on code or features)
• Dependencies (update dependencies and changes associated, has no impact on code or features)
• Unit Tests (work on Unit Tests, has no impact on code or features)
• Documentation (changes or updates in the documentation, has no impact on code or features)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[Ellerbach]

Thanks for the fix!

[nfbot]

@cammurray thank you again for your contribution! 🙏😄

.NET nanoFramework it's all about community involvement and no contribution is too small.
We would like to invite you to join the project Contributors list.

Please edit it and add an entry with your GitHub user in the appropriate location (names sorted alphabetically):

  <tr>
    <td><img src="https://github.com/cammurray.png?size=50&" height="50" width="50" ></td>
    <td><a href="https://github.com/cammurray">Cam Murray</a></td>
  </tr>

(feel free to adjust your name, if it's not correct)