doc: application: clarify a pronoun antecedent

   target_sources(app PRIVATE src/main.c)

In the above code, an existing library target named ``app`` is

configured to include the source file :file:`src/main.c`. The ``PRIVATE``

In the above :file:`CMakeLists.txt`, an existing library target named ``app``

is configured to include the source file :file:`src/main.c`. The ``PRIVATE``

keyword indicates that we are modifying the internals of how the library is

being built. Using the keyword ``PUBLIC`` would modify how other

libraries that link with app are built. In this case, using ``PUBLIC``