doc/dev: s/master/main/ in basic workflow

doc/dev: s/master/main/ in basic workflow

This PR changes "master" to "main" in the
basic_workflow.rst file. I have even changed
"master" to "main" in some terminal output from
several years ago. This isn't historically ac-
curate, of course, but my hope is that this change
will prevent someone in the future from being con-
fused about why an antiquated branch name is ref-
erred to.

Signed-off-by: Zac Dover <zac.dover@gmail.com>
(cherry picked from commit f5cfc22445a3e454e572873cdcfbc1852a38aa17)