Use egg:swift3#swift3 instead of egg:swift#swift3

From the bugreport:
"I also want to point that "use = egg:swift#swift3" is wrongly stated in
the doc. It has to be "use = egg:swift3#swift3" - as swift3 is not a
module of swift but of swift3."

The above change is confirmed by
https://github.com/fujita/swift3/blob/master/README.md

Change-Id: Icc190cf01586f3d41faaf57a2771dc1af5883876
Closes-Bug: #1209254
Merge "Use egg:swift3#swift3 instead of egg:swift#swift3"