README.md: replace hardcoded path to command substitution

Signed-off-by: KOSHIKAWA Kenichi <reishoku.misc@pm.me>

Author: reishoku

README.md

@@ -24,11 +24,7 @@ Download the latest stable [release][release] for use on your desktop or server.
 ```shell
 brew install todo-txt
 
-# For macOS on x86 CPU 
-cp -n /usr/local/opt/todo-txt/todo.cfg ~/.todo.cfg
-
-# For macOS on arm CPU
-cp -n /opt/homebrew/opt/todo-txt/todo.cfg ~/.todo.cfg
+cp -n $(brew --prefix)/opt/todo-txt/todo.cfg ~/.todo.cfg
 ```
 
 **Note**: The `-n` flag for `cp` makes sure you do not overwrite an existing file.