docs: stabilisation -> stabilization

We generally prefer US English in the docs over UK English.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6894>
diff --git a/docs/releasing.rst b/docs/releasing.rst
index 39db085..ee41dd4 100644
--- a/docs/releasing.rst
+++ b/docs/releasing.rst
@@ -165,7 +165,7 @@
 --------------------
 
 A branchpoint is made such that new development can continue in parallel
-to stabilisation and bugfixing.
+to stabilization and bugfixing.
 
 .. note::