Actually, he didn't. The text does not specify "Afghanistan".
Ron Paul voted for the AUMF against the 9-11 terrorists because "To declare war against a group that is not a country makes the clear declaration of war more complex.... The proposed resolution is the only option we're offered and doing nothing is unthinkable." Ron Paul, http://www.house.gov/paul/tst/tst2001/tst091701.htm
However, while it may not be possible to declare war against "a group that is not a country", Iraq is a Country -- and the Constitution provides clear details on how the United States is to go about making War on a Country. Specifically, Congress is to issue a Declaration of War, as Ron Paul advocated.
"The proposed resolution is the only option we're offered and doing nothing is unthinkable."
The resolution didn't specifically mention Afghanistan, but everybody knew at the time that the Taliban government in Afghanistan was harboring al Qaida and the US was going to attack Afghanistan. However, the proposed resolution on Iraq was the only option Congress was offered in that instance, and Ron Paul gave a speech in Congress where he made clear he was against going to war against Saddam Hussein.
"However, while it may not be possible to declare war against "a group that is not a country", Iraq is a Country -- and the Constitution provides clear details on how the United States is to go about making War on a Country."
Then what about Afghanistan? It is a country too.