[spec] Fix typo

document/core/valid/instructions.rst

@@ -408,7 +408,7 @@ The following auxiliary function denotes the number of lanes in a vector shape,
    \frac{
      \laneidx < \dim(\shape)
    }{
-     C \vdashinstr t\K{x}N\K{.}\EXTRACTLANE\K{\_}\sx^?~\laneidx : [\V128] \to [\unpacked(\shape)]
+     C \vdashinstr \shape\K{.}\EXTRACTLANE\K{\_}\sx^?~\laneidx : [\V128] \to [\unpacked(\shape)]
    }