fix(program_units_and_procedures.md): edit forall referer

Extend previously used referer about `forall` across an other
file.  This presumes back ticks can be dropped when referring
to a specific section title.

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

Author: nbehrnd

source/learn/f95_features/program_units_and_procedures.md

@@ -354,7 +354,7 @@ Here, we note the `result` clause and termination test.
 This is a feature for parallel computing.
 
 In
-[the `forall` statement and construct](forall-statement),
+[the `forall` statement and construct](array_handling.md#the-forall-statement-and-construct),
 any side effects in a function can impede optimization on
 a parallel processor the order of execution of the assignments could
 affect the results. To control this situation, we add the `pure` keyword