[fix] Fix doc to build web extension

doc/build_web_extension.md

@@ -16,7 +16,7 @@ After that you should be able to start the application using `npm start`or `yarn
 - To create a uncompressed extension, use :
   ```bash
      cd cesium
-     gulp webExtensionCopyFiles --release
+     gulp webExtCompile --release
   ```
   
   The uncompressed web extension will be visible `dist/web/ext`

gulpfile.js

@@ -812,6 +812,7 @@ exports.build = build;
 exports.webBuild = webBuild;
 exports['build:web'] = exports.webBuild; // Alias
 
+exports.webExtCompile = webExtCompile;
 exports.webExtBuild = webExtBuild;
 exports['build:webExt'] = exports.webBuild; // Alias