cmake: Fix typo by replacing zephyr_library_add_sources
There is no function called zephyr_library_add_sources().  This must be
zephyr_library_sources().
@@ -436,7 +436,7 @@ endmacro()
# ZEPHYR_MODULE/drivers/entropy/CMakeLists.txt
# with content:
# zephyr_library_amend()
-# zephyr_library_add_sources(...)
+# zephyr_library_sources(...)
#
# It is also possible to use generator expression when amending to Zephyr
# libraries.
