provision: Fix source command on Windows

[QEDK]

Currently, the provision script gives us something like this on Windows:

Run the following to enter into the virtualenv:

  source C:\Users\qedk\python-zulip-api\zulip-api-py3-venv\Scripts\activate

However, bash does not work with backslashes and will give us an error saying No such file or directory.
This commit addresses that so we get the correct command with the forward slashes:

Run the following to enter into the virtualenv:

  source C:/Users/qedk/python-zulip-api/zulip-api-py3-venv/Scripts/activate

[showell]

Merged, thanks @QEDK!