Fix typo in pants bash script.

Forgot to update the review at https://reviews.apache.org/r/27062/
to match my local branch.

Testing Done:
% rm -fr build-support/pants.venv
% ./pants
% ls -l build-support/pants.venv
total 16
drwxrwxr-x 2 ksweeney ksweeney 4096 Oct 22 17:37 bin
-rw-rw-r-- 1 ksweeney ksweeney    7 Oct 22 17:37 BOOTSTRAPPED
drwxrwxr-x 2 ksweeney ksweeney 4096 Oct 22 17:36 include
drwxrwxr-x 3 ksweeney ksweeney 4096 Oct 22 17:36 lib

Reviewed at https://reviews.apache.org/r/27062/