fix(program_units_and_procedures.md): expand reference

The reference to file `pointers.md` now is expanded to the
complete file name with file extension.

Signed-off-by: Norwid Behrnd <nbehrnd@yahoo.com>

Author: nbehrnd

source/learn/f95_features/program_units_and_procedures.md

@@ -254,7 +254,7 @@ An explicit interface is obligatory for
 
 - optional and keyword arguments;
 - `pointer` and `target` arguments (see
-  [Pointers](pointers));
+  [Pointers](pointers.md));
 - `pointer` function result;
 - new-style array arguments and array functions
   ([Array handling](array_handling)).