Update runtime.txt

[ErfanRFZ]

its doesn't work I think this change will fix it. error: -----> Using Python version specified in runtime.txt ! Requested runtime (python-3.7.6) is not available for this stack (heroku-20). ! Aborting. More info: https://devcenter.heroku.com/articles/python-support ! Push rejected, failed to compile Python app. ! Push failed