[gdb/contrib] Add two rules in common-misspellings.txt

Eli mentioned [1] that given that we use US English spelling in our
documentation, we should use "behavior" instead of "behaviour".

In wikipedia-common-misspellings.txt there's a rule:
...
behavour->behavior, behaviour
...
which leaves this as a choice.

Add an overriding rule to hardcode the choice to common-misspellings.txt:
...
behavour->behavior
...
and add a rule to rewrite behaviour into behavior:
...
behaviour->behavior
...
and re-run spellcheck.sh on gdb*.

Tested on x86_64-linux.

[1] https://sourceware.org/pipermail/gdb-patches/2024-November/213371.html

gdb/python/py-lazy-string.c

@@ -264,7 +264,7 @@ stpy_lazy_string_elt_type (lazy_string_object *lazy)
     case TYPE_CODE_ARRAY:
       return check_typedef (realtype->target_type ());
     default:
-      /* This is done to preserve existing behaviour.  PR 20769.
+      /* This is done to preserve existing behavior.  PR 20769.
 	 E.g., gdb.parse_and_eval("my_int_variable").lazy_string().type.  */
       return realtype;
     }