fix(expressions_and_assignments.md): edit cross link

Following the previously spot pattern, the link across to file
`language_elements.md`, section "Basics" is edited.

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

Author: nbehrnd

source/learn/f95_features/expressions_and_assignments.md

@@ -128,7 +128,7 @@ end interface
 
 The string concatenation function is a more elaborated version of that
 shown already in
-[Basics](Basics).
+[Basics](language_elements.md#basics).
 Note that
 in order to handle the error condition that arises when the two strings
 together exceed the preset 80-character limit, it would be safer to use