Update document/core/exec/instructions.rst

Author: rossberg

document/core/exec/instructions.rst

@@ -210,9 +210,8 @@ Reference Instructions
    \end{array}
 
 .. note::
-   This is the reduction rule for |REFNULL| |TYPEIDX|.
-   No formal reduction rule is required for |REFNULL| |ABSHEAPTYPE|,
-   since the instruction is already a :ref:`value <syntax-val>`.
+   No formal reduction rule is required for the case |REFNULL| |ABSHEAPTYPE|,
+   since the instruction form is already a :ref:`value <syntax-val>`.
 
 
 .. _exec-ref.func: