Add correct license file name

[jordansissel]

After running the generator, a file "LICENSE.txt" exists in the repo.

During make setup, the target git-init runs and tries to git add LICENCE. This fails and interrupts the setup.

The error you get is:

git add LICENSE
fatal: pathspec 'LICENSE' did not match any files
Makefile:29: recipe for target 'git-init' failed

This patch resolves the error.

[ruflin]

Thanks. We recently renamed the LICENSE file but be consistent with the Beats repo but forgot to also rename this part :-(