Fix typos in SIMD exec/instructions

Author: ngzhian

document/core/exec/instructions.rst

@@ -394,7 +394,7 @@ Most vector instructions are defined in terms of generic numeric operators appli
 .. math::
    \begin{array}{l}
    \begin{array}{lcl@{\qquad}l}
-   (\V128\K{.}\VCONST~c_1)~(\V128\K{.}\VCONST~c_2)~\V128\K{.}\SWIZZLE &\stepto& (\V128\K{.}\VCONST~c')
+   (\V128\K{.}\VCONST~c_1)~(\V128\K{.}\VCONST~c_2)~\I8X16\K{.}\SWIZZLE &\stepto& (\V128\K{.}\VCONST~c')
    \end{array}
    \\ \qquad
      \begin{array}[t]{@{}r@{~}l@{}}
@@ -431,7 +431,7 @@ Most vector instructions are defined in terms of generic numeric operators appli
 .. math::
    \begin{array}{l}
    \begin{array}{lcl@{\qquad}l}
-   (\V128\K{.}\VCONST~c_1)~(\V128\K{.}\VCONST~c_2)~\V128\K{.}\SHUFFLE~x^\ast &\stepto& (\V128\K{.}\VCONST~c)
+   (\V128\K{.}\VCONST~c_1)~(\V128\K{.}\VCONST~c_2)~\I8X16\K{.}\SHUFFLE~x^\ast &\stepto& (\V128\K{.}\VCONST~c)
    \end{array}
    \\ \qquad
      \begin{array}[t]{@{}r@{~}l@{}}