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/readme-mbed.md b/docs/readme-mbed.md
index 189c7bf..2162678 100644
--- a/docs/readme-mbed.md
+++ b/docs/readme-mbed.md
@@ -16,7 +16,7 @@
 * `"mcuboot.max-img-sectors"`: maximum number of sectors, should be at least the number of sectors in each slot
 * `"target.restrict_size"`: the maximum size of the bootloader, such that it does not overlap with the primary slot
 
-More configurations such as signing algorithm, slot swapping, etc. can be found in [mbed_lib.json](https://github.com/mcu-tools/mcuboot/tree/master/boot/mbed/mbed_lib.json). Please note that certain features are not currently supported.
+More configurations such as signing algorithm, slot swapping, etc. can be found in [mbed_lib.json](https://github.com/mcu-tools/mcuboot/tree/main/boot/mbed/mbed_lib.json). Please note that certain features are not currently supported.
 
 ### Providing a secondary slot