[spec] Fix definition of iextend (#1696)

Author: rossberg

document/core/exec/numerics.rst

@@ -715,11 +715,13 @@ The integer result of predicates -- i.e., :ref:`tests <syntax-testop>` and :ref:
 :math:`\iextendMs_N(i)`
 .......................
 
-* Return :math:`\extends_{M,N}(i)`.
+* Let :math:`j` be the result of computing :math:`\wrap_{N,M}(i)`.
+
+* Return :math:`\extends_{M,N}(j)`.
 
 .. math::
    \begin{array}{lll@{\qquad}l}
-   \iextendMs_{N}(i) &=& \extends_{M,N}(i) \\
+   \iextendMs_{N}(i) &=& \extends_{M,N}(\wrap_{N,M}(i)) \\
    \end{array}