Replace reference to master

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/BUGS.md b/BUGS.md
index 47bde07..a65c606 100644
--- a/BUGS.md
+++ b/BUGS.md
@@ -7,7 +7,7 @@
 If you think you've found a bug in Mbed TLS, please follow these steps:
 
 1. Make sure you're using the latest version of a
-   [maintained branch](BRANCHES.md): `master`, `development`,
+   [maintained branch](BRANCHES.md): `main`, `development`,
    or a long-time support branch.
 2. Check [GitHub](https://github.com/Mbed-TLS/mbedtls/issues) to see if
    your issue has already been reported. If not, …