Migrate master->main

Change references in CI and docs from 'master' to 'main' as the primary
branch has been renamed.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/docs/release.md b/docs/release.md
index ff00cec..90c32b5 100644
--- a/docs/release.md
+++ b/docs/release.md
@@ -32,7 +32,7 @@
 time, and allow testing to happen.
 
 During the time between rc1 and the final release, the only changes
-that should be merged into master are those to fix bugs found in the
+that should be merged into main are those to fix bugs found in the
 rc and Mynewt metadata as described in the next section.
 
 ## imgtool release
@@ -81,7 +81,7 @@
 At this point, the tag can be pushed to github to make the actual
 release happen:
 ``` bash
-git push origin HEAD:refs/heads/master
+git push origin HEAD:refs/heads/main
 git push origin va.b.c-rcn
 ```