Alter wording of example Skip Nav link to improve pronunciation

Screen reading applications, including Apple's VoiceOver, improperly pronounce the word "content" in "Skip to content" as an adjective rather than a noun because of insufficient context. "Skip to main content" is pronounced correctly. 

See #6 and Guideline #10 at the following site for more information:
http://redish.net/content/papers/interactions.html