Fix the URL for the sass_api package on pub.dev (#1435)

Author: stof

README.md

@@ -137,7 +137,7 @@ This provides access to the Sass AST and APIs for resolving Sass loads without
 running a full compilation. It's separated out into its own package so that it
 can increase its version number independently of the main `sass` package.
 
-[`sass_api` package]: https://pub.dev/package/sass_api
+[`sass_api` package]: https://pub.dev/packages/sass_api
 
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