leverage browser caching warning on google pagespeed

After I have activated the nice Google Analytics+ Plugin I receve the following message on google pagespeed:

Leverage browser caching
Setting an expiry date or a maximum age in the HTTP headers for static resources instructs the browser to load previously downloaded resources from locLeverage browser caching
Setting an expiry date or a maximum age in the HTTP headers for static resources instructs the browser to load previously downloaded resources from local disk rather than over the network.
Leverage browser caching for the following cacheable resources:

http://www.google-analytics.com/ga.js (2 hours)

Can you tell me what entry I have to put in .htaccess database in the browoser caching section to eliminate this?
Or do you have another possibility to solve the problem?

Thank you very much and greetings, Marc

Edit: Browser Caching for .js files is activated and works fine, other then this file, which is an external link.