Replace "main" with "top level" in goto model explaination

Main suggests that there are other entities which hold goto, rather than
expressing that all the other data structures are contained within the
model.

Author: thomasspriggs

doc/architectural/central-data-structures.md

@@ -6,8 +6,8 @@ CBMC and the assorted CProver tools.
 
 ## goto_modelt
 
-The `goto_modelt` is the main data structure that CBMC (and the other tools) use
-for holding the GOTO intermediate representation.
+The `goto_modelt` is the top-level data structure that CBMC (and the other
+tools) use for holding the GOTO intermediate representation.
 
 A `goto_modelt` is effectively a pair, consisting of: