Update documentation/arduino_libs.md

Be explicit on how to add the source file to the build

Co-authored-by: Mike Szczys <[email protected]>

Author: DhruvaG2000

documentation/arduino_libs.md

@@ -19,7 +19,7 @@ Paste your library's source files like ``ADXL345.h`` and ``ADXL345.cpp`` in your
 
 ## Update CMakeLists
 As we can see, there is a Top-level CMakelists which needs to be updated with your external library's source files.
-For example refer to the ADXL345 demo has ``ADXL345.cpp`` in it's CMakeLists.
+For example, we add `target_sources(app PRIVATE src/ADXL345.cpp)`  to CMakeLists.txt.
 
 ## Update main.cpp
 Finally, paste your required code using the library in ``main.c`` and include that library's header using