[LangRef][VP] Correct mask type in vp.slice documentation

[frasercrmck]

Summary

The mask type for the llvm.experimental.vp.splice intrinsics must have
the same number of elements as the result type.

llvm/docs/LangRef.rst

 
 Syntax:
 """""""
 This is an overloaded intrinsic.
 
 ::
 
       declare <2 x double> @llvm.experimental.vp.splice.v2f64(<2 x double> %vec1, <2 x double> %vec2, i32 %imm, <2 x i1> %mask, i32 %evl1, i32 %evl2)
-      declare <vscale x 4 x i32> @llvm.experimental.vp.splice.nxv4i32(<vscale x 4 x i32> %vec1, <vscale x 4 x i32> %vec2, i32 %imm, <2 x i1> %mask i32 %evl1, i32 %evl2)
+      declare <vscale x 4 x i32> @llvm.experimental.vp.splice.nxv4i32(<vscale x 4 x i32> %vec1, <vscale x 4 x i32> %vec2, i32 %imm, <vscale x 4 x i1> %mask, i32 %evl1, i32 %evl2)
 
 Overview:
 """""""""
 
 The '``llvm.experimental.vp.splice.*``' intrinsic is the vector length
 predicated version of the '``llvm.experimental.vector.splice.*``' intrinsic.
 
 Arguments: