FIX: small typo in an example using the "Referer" header

FIX: small typo in an example using the "Referer" header

It was reported that an example was manipulating a "Referrer" header instead
of the known "Referer" one. Even if it's an example wich doesn't break things,
the typo can be fixed.

The fix should be backported in 1.4/1.5/1.6 branches.