[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/source.c

@@ -758,7 +758,7 @@ prepare_path_for_appending (const char *path)
    using mode MODE in the calls to open.  You cannot use this function to
    create files (O_CREAT).
 
-   OPTS specifies the function behaviour in specific cases.
+   OPTS specifies the function behavior in specific cases.
 
    If OPF_TRY_CWD_FIRST, try to open ./STRING before searching PATH.
    (ie pretend the first element of PATH is ".").  This also indicates
@@ -949,7 +949,7 @@ done:
 }
 
 
-/* This is essentially a convenience, for clients that want the behaviour
+/* This is essentially a convenience, for clients that want the behavior
    of openp, using source_path, but that really don't want the file to be
    opened but want instead just to know what the full pathname is (as
    qualified against source_path).