Make the FetchContent name be lower case (#25)

This brings the code in-line with what is done in the Pico SDK and
seems to be required in order for pico_extras_SOURCE_PATH to be set.

Fixes #24

Co-authored-by: Graham Sanderson <graham.sanderson@raspberrypi.com>

external/pico_extras_import.cmake

@@ -26,13 +26,13 @@ if (NOT PICO_EXTRAS_PATH)
             get_filename_component(FETCHCONTENT_BASE_DIR "${PICO_EXTRAS_FETCH_FROM_GIT_PATH}" REALPATH BASE_DIR "${CMAKE_SOURCE_DIR}")
         endif ()
         FetchContent_Declare(
-                PICO_EXTRAS
+                pico_extras
                 GIT_REPOSITORY https://github.com/raspberrypi/pico-extras
                 GIT_TAG master
         )
-        if (NOT PICO_EXTRAS)
-            message("Downloading PICO EXTRAS")
-            FetchContent_Populate(PICO_EXTRAS)
+        if (NOT pico_extras)
+            message("Downloading Raspberry Pi Pico Extras")
+            FetchContent_Populate(pico_extras)
             set(PICO_EXTRAS_PATH ${pico_extras_SOURCE_DIR})
         endif ()
         set(FETCHCONTENT_BASE_DIR ${FETCHCONTENT_BASE_DIR_SAVE})