🤖 docs: Correctly import regenerator/runtime.

This is needed because the new ESM import syntax requires full paths.

These changes were automatically generated by a transform whose code can be found at:
  - https://github.com/aureooms/rejuvenate/blob/eb1b209cd7aa675a642d48b2a788c2c6112779f7/src/transforms/docs:import-regenerator-runtime-the-esm-way.js
Please contact the author of the transform if you believe there was an error.

Author: a-flying-potato

doc/manual/usage.md

@@ -4,7 +4,7 @@ The code needs a ES2015+ polyfill to work, for example
 ```js
 require( 'regenerator-runtime/runtime' ) ;
 // or
-import 'regenerator-runtime/runtime' ;
+import 'regenerator-runtime/runtime.js' ;
 ```
 
 Then