fix(program_units_and_procedures.md): edit markup on keyword

Because the sinus function was not clearly marked as such in
the original document, the translation did not recognize this
omission earlier.

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

Author: nbehrnd

source/learn/f95_features/program_units_and_procedures.md

@@ -292,7 +292,7 @@ interface gamma
 end interface
 ```
 
-We can use existing names, e.g. SIN, and the compiler sorts out the
+We can use existing names, e.g. `sin`, and the compiler sorts out the
 correct association.
 
 We have already seen the use of interface blocks for defined operators