Update docs: `Makefile` -> `rules.mk`

I think this was a typo, or perhaps an older way to handle it.

docs/getting_started_introduction.md

@@ -11,7 +11,7 @@ QMK is a fork of [Jun Wako](https://github.com/tmk)'s [tmk_keyboard](https://git
 Within the folder `keyboards` and its subfolder `handwired` is a directory for each keyboard project, for example `qmk_firmware/keyboards/clueboard`. Within it you'll find the following structure:
 
 * `keymaps/`: Different keymaps that can be built
-* `rules.mk`: The file that sets the default "make" options. Do not edit this file directly, instead use a keymap specific `Makefile`
+* `rules.mk`: The file that sets the default "make" options. Do not edit this file directly, instead use a keymap specific `rules.mk`.
 * `config.h`: The file that sets the default compile time options. Do not edit this file directly, instead use a keymap specific `config.h`.
 
 ### Keymap Structure