[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-framefilter.c

@@ -762,7 +762,7 @@ py_print_frame (PyObject *filter, frame_filter_flags flags,
      default value for the backtrace command (see the call to print_frame_info
      in backtrace_command_1).
      Having the same default ensures that 'bt' and 'bt no-filters'
-     have the same behaviour when some filters exist but do not apply
+     have the same behavior when some filters exist but do not apply
      to a frame.  */
   enum print_what print_what
     = out->is_mi_like_p () ? LOC_AND_ADDRESS : LOCATION;