fix: Update documentation to reference 4.x pdf.js worker extension

In v4.x of PDF.js the build files have changed from .js to .mjs

This commit updates the example usage in the README from .js to .mjs to closer align to the most common use case for this section

README.md

@@ -511,9 +511,9 @@ onFileSelected() {
 
 By default the `worker` is loaded from `cdn.jsdelivr.net`.
 
-In your code update `path` to the worker to be for example `/pdf.worker.js`
+In your code update `path` to the worker to be for example `/pdf.worker.mjs`
 ```typescript
-(window as any).pdfWorkerSrc = '/pdf.worker.js';
+(window as any).pdfWorkerSrc = '/pdf.worker.mjs';
 ```
 
 *This should be set before `pdf-viewer` component is rendered.*
@@ -524,7 +524,7 @@ but using different versions of pdf.worker, support has been added.  You can do
 above, except that you can append the specific version of pdfjs required and override the
 custom path *just for that version*.  This way setting the global window var won't conflict.
 ```typescript
-(window as any)["pdfWorkerSrc2.14.305"] = '/pdf.worker.js';
+(window as any)["pdfWorkerSrc2.14.305"] = '/pdf.worker.mjs';
 ```
 
 ## Search in the PDF