[spec] Change misleading wording in note (#1855)

Author: rossberg

document/core/binary/modules.rst

@@ -568,5 +568,5 @@ where for each :math:`t_i^\ast, e_i` in :math:`\X{code}^n`,
    The version of the WebAssembly binary format may increase in the future
    if backward-incompatible changes have to be made to the format.
    However, such changes are expected to occur very infrequently, if ever.
-   The binary format is intended to be forward-compatible,
-   such that future extensions can be made without incrementing its version.
+   The binary format is intended to be extensible,
+   such that future features can be added without incrementing its version.