Fix python-environment-exists-p

Previous version was accidentally working.

Author: tkf

python-environment.el

@@ -153,7 +153,7 @@ For reason, see `python-environment-run-block'"
 (defun python-environment-exists-p (&optional root)
  "Return non-`nil' if virtualenv at ROOT exists.
 See `python-environment-make' for how ROOT is interpreted."
- (let ((bin (python-environment-bin root)))
+ (let ((bin (python-environment-bin "python" root)))
  (and bin (file-exists-p bin))))
 
 (defun python-environment--existing (root &rest paths)