[spec] Fix typo (#1587)

Author: bongjunj

document/core/text/instructions.rst

@@ -165,7 +165,7 @@ Reference Instructions
    \production{instruction} & \Tplaininstr_I &::=& \dots \\ &&|&
      \text{ref.null}~~t{:}\Theaptype &\Rightarrow& \REFNULL~t \\ &&|&
      \text{ref.is\_null} &\Rightarrow& \REFISNULL \\ &&|&
-     \text{ref.func}~~x{:}\Tfuncidx &\Rightarrow& \REFFUNC~x \\ &&|&
+     \text{ref.func}~~x{:}\Tfuncidx &\Rightarrow& \REFFUNC~x \\
    \end{array}
 
 
@@ -891,7 +891,7 @@ Vector constant instructions have a mandatory :ref:`shape <syntax-vec-shape>` de
      \text{f64x2.convert\_low\_i32x4\_s} &\Rightarrow& \F64X2.\VCONVERT\K{\_low\_i32x4\_s}\\  &&|&
      \text{f64x2.convert\_low\_i32x4\_u} &\Rightarrow& \F64X2.\VCONVERT\K{\_low\_i32x4\_u}\\ &&|&
      \text{f32x4.demote\_f64x2\_zero} &\Rightarrow& \F32X4.\VDEMOTE\K{\_f64x2\_zero}\\ &&|&
-     \text{f64x2.promote\_low\_f32x4} &\Rightarrow& \F64X2.\VPROMOTE\K{\_low\_f32x4}\\ &&|&
+     \text{f64x2.promote\_low\_f32x4} &\Rightarrow& \F64X2.\VPROMOTE\K{\_low\_f32x4}\\
    \end{array}