fixup! doc/guides/porting-boards: fix reference link

Author: crasbe

doc/guides/advanced_tutorials/porting_boards.md

@@ -345,7 +345,7 @@ by many of the nRF52 based boards from Adafruit, you simply have to add the
 following line to the `Makefile.dep` of your board. Everything else
 will be automatically included by the build system.
 
-```mk
+```makefile
 USEMODULE += boards_common_adafruit-nrf52-bootloader
 ```
 
@@ -360,7 +360,7 @@ community.
 If you want to use common makefiles, include them at the end of the specific
 `Makefile`, e.g. for a `Makefile.features`:
 
-```mk
+```makefile
 CPU = foo
 CPU_MODEL = foobar
 # Put defined MCU peripherals here (in alphabetical order)
@@ -374,7 +374,7 @@ If the common code includes source files, it might be necessary
 to explicitly include the directory in your `Makefile` so the Make system
 finds all the necessary files:
 
-```mk
+```makefile
 MODULE = board
 
 DIRS += $(RIOTBOARD)/common/myCommonFolder
@@ -414,7 +414,7 @@ RIOT
 The main `Makefile` defines the module name for the common board module and
 should follow the general naming scheme of `boards_common_awesome-common-stuff`.
 
-```Makefile
+```makefile
 MODULE = boards_common_adafruit-nrf52-bootloader
 
 include $(RIOTBASE)/Makefile.base