[spec] Fix typo in validation of call_ref (#1939)

Author: 4y8

document/core/valid/instructions.rst

@@ -2455,7 +2455,7 @@ Control Instructions
 
 * The type :math:`C.\CTYPES[x]` must be defined in the context.
 
-* The :ref:`expansion <aux-expand-deftype>` of :math:`C.\CFUNCS[x]` must be a :ref:`function type <syntax-functype>` :math:`\TFUNC~[t_1^\ast] \toF [t_2^\ast]`.
+* The :ref:`expansion <aux-expand-deftype>` of :math:`C.\CTYPES[x]` must be a :ref:`function type <syntax-functype>` :math:`\TFUNC~[t_1^\ast] \toF [t_2^\ast]`.
 
 * Then the instruction is valid with type :math:`[t_1^\ast~(\REF~\NULL~x)] \to [t_2^\ast]`.