Fixed the grammatical error in the readme file

[hemangtk]

Changed MicroCode is a icon-based to MicroCode is an icon-based

By submitting this pull request I confirm I've read and complied with the below requirements 🖖

• [ x ] I have read and understood the Contribution Guidelines
• [ x ] I am making an individual pull request for each suggestion
• [ x ] I have used the correct spelling and capitalisation for BBC micro:bit, micro:bit, and Micro:bit Educational Foundation (resource title could be excluded if there is a good reason)
• [ x ] The content linked is in English, or it contains an English translation
• [ x ] I have added the new entry to the bottom of its the category
• [ x ] I have used the following format:

[carlosperate]

Thanks @hemangtk!