Use https instead of ssh for submodule (#14)

some workflows don't support ssh

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "lib/esp-serial-flasher"]
 	path = lib/esp-serial-flasher
-	url = git@github.com:0xchocolate/esp-serial-flasher.git
+	url = https://github.com/0xchocolate/esp-serial-flasher.git