docs/CODING_STYLE.md: add src prefix to path
## File format
-Use [lv_misc/lv_templ.c](https://github.com/littlevgl/lvgl/blob/master/lv_misc/lv_templ.c) and [lv_misc/lv_templ.h](https://github.com/littlevgl/lvgl/blob/master/lv_misc/lv_templ.h)
+Use [lv_misc/lv_templ.c](https://github.com/littlevgl/lvgl/blob/master/src/lv_misc/lv_templ.c) and [lv_misc/lv_templ.h](https://github.com/littlevgl/lvgl/blob/master/src/lv_misc/lv_templ.h)
## Naming conventions
* Words are separated by '_'
