Update manual-rum-browser-instrumentation.rst

Added importing splunk/otel-web for the "set the user id on all spans" example

Author: jelkhouri-splunk

gdi/get-data-in/rum/browser/manual-rum-browser-instrumentation.rst

@@ -48,6 +48,7 @@ Set the user ID on all spans
 The following example shows how to set the user ID globally:
 
 .. code-block:: javascript
+   import SplunkRum from '@splunk/otel-web';
 
    SplunkRum.setGlobalAttributes({
       'enduser.id': 'Test User'