doc: update README.md

Author: jaywcjlove

README.md

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-A tool that can convert any SVG icon into a [custom SF symbol](https://developer.apple.com/documentation/uikit/uiimage/creating_custom_symbol_images_for_your_app). Your customized SF symbol can be imported into Xcode and utilized in any project based on UIKit or SwiftUI.
+A tool that can convert any SVG icon into a [Custom SF symbol](https://developer.apple.com/documentation/uikit/uiimage/creating_custom_symbol_images_for_your_app). Your customized SF symbol can be imported into Xcode and utilized in any project based on UIKit or SwiftUI.
 
 ![Create Custom Symbols screenshots-1](./assets/screenshots-1.png)