fix(program_units_and_procedures.md): edit referer

To refer to a chapter in the same markdown file, the referer
has to be in lower case.

Signed-off-by: Norwid Behrnd <[email protected]>

Author: nbehrnd

source/learn/f95_features/program_units_and_procedures.md

@@ -297,7 +297,7 @@ correct association.
 
 We have already seen the use of interface blocks for defined operators
 and assignment (see
-[Modules](Modules)).
+[Modules](modules)).
 
 ## Recursion