portable-RP2040: Fix typo in README.md (#559)

Replace "import" with "include" in cmake code sample.
diff --git a/portable/ThirdParty/GCC/RP2040/README.md b/portable/ThirdParty/GCC/RP2040/README.md
index 5a0fe4d..c50cb4e 100644
--- a/portable/ThirdParty/GCC/RP2040/README.md
+++ b/portable/ThirdParty/GCC/RP2040/README.md
@@ -14,7 +14,7 @@
 add the following in your `CMakeLists.txt`:
 
 ```cmake
-import(FreeRTOS_Kernel_import.cmake)
+include(FreeRTOS_Kernel_import.cmake)
 ```
 
 This will locate the FreeRTOS kernel if it is a direct sub-module of your project, or if you provide the 
@@ -39,4 +39,4 @@
 
 ## Known Limitations
 
-- Tickless idle has not currently been tested, and is likely non-functional
\ No newline at end of file
+- Tickless idle has not currently been tested, and is likely non-functional