doc: update default branch name in Makefile

PR-URL: #43441
Refs: #33864
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: LiviaMedeiros <livia@cirno.name>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

Makefile

@@ -1429,8 +1429,8 @@ CLANG_FORMAT_START ?= HEAD
 #  $ make format-cpp
 # To format HEAD~1...HEAD (latest commit):
 #  $ CLANG_FORMAT_START=`git rev-parse HEAD~1` make format-cpp
-# To format diff between master and current branch head (master...HEAD):
-#  $ CLANG_FORMAT_START=master make format-cpp
+# To format diff between main and current branch head (main...HEAD):
+#  $ CLANG_FORMAT_START=main make format-cpp
 format-cpp: ## Format C++ diff from $CLANG_FORMAT_START to current changes
 ifneq ("","$(wildcard tools/clang-format/node_modules/)")
 	$(info Formatting C++ diff from $(CLANG_FORMAT_START)..)