Fix signed convert subscript (WebAssembly#1248)

Fixed WebAssembly#1247.

Author: ngzhian

document/core/exec/numerics.rst

@@ -1603,7 +1603,7 @@ Conversions
 
 .. math::
    \begin{array}{lll@{\qquad}l}
-   \convertu_{M,N}(i) &=& \ieee_N(\signed_M(i)) \\
+   \converts_{M,N}(i) &=& \ieee_N(\signed_M(i)) \\
    \end{array}