Correct "list of GOTO functions" to "collection"

Because the data structure used is not a list and because we should
avoid being specific here in order to maintain a high level of
abstraction.

Author: thomasspriggs

doc/architectural/central-data-structures.md

@@ -56,7 +56,7 @@ erDiagram
 
 A `goto_modelt` is effectively a pair, consisting of:
 
-* A list of GOTO functions,
+* A collection of GOTO functions.
 * A symbol table containing symbol references for the symbols contained in the GOTO functions.
 
 In pseudocode, the type looks this: