docs: reference THEIA_WEBVIEW_EXTERNAL_ENDPOINT

The 'THEIA_WEBVIEW_EXTERNAL_ENDPOINT' environment variable configures
the webview host pattern. The '@theia/plugins-ext' README referred to
'THEIA_WEBVIEW_ENDPOINT_PATTERN' in error. This has now been corrected.

packages/plugin-ext/README.md

@@ -20,7 +20,7 @@ The implementation is inspired from: https://blog.mattbierner.com/vscode-webview
 
 ## Environment Variables
 
-- `THEIA_WEBVIEW_ENDPOINT_PATTERN`
+- `THEIA_WEBVIEW_EXTERNAL_ENDPOINT`
 
   A string pattern possibly containing `{{uuid}}` and `{{hostname}}` which will be replaced. This is the host for which the `webviews` will be served on.
   It is a good practice to host the `webview` handlers on a sub-domain as it is more secure.
@@ -30,7 +30,7 @@ The implementation is inspired from: https://blog.mattbierner.com/vscode-webview
 
 - Potentially Insecure Host Pattern
 
-  When you change the host pattern via the `THEIA_WEBVIEW_ENDPOINT_PATTERN` environment variable warning will be emitted both from the frontend and from the backend.
+  When you change the host pattern via the `THEIA_WEBVIEW_EXTERNAL_ENDPOINT` environment variable warning will be emitted both from the frontend and from the backend.
   You can disable those warnings by setting `warnOnPotentiallyInsecureHostPattern: false` in the appropriate application configurations in your application's `package.json`.
 
 ## Runtime System Plugin Resolvement