[llvm][doc] Fix return type of vector.histogram in LangRef examples (NFC) (llvm#113172)

The `@llvm.experimental.vector.histogram.add` returns void.

Author: MacDue

llvm/docs/LangRef.rst

@@ -19925,8 +19925,8 @@ More update operation types may be added in the future.
 
 ::
 
-    declare <8 x i32> @llvm.experimental.vector.histogram.add.v8p0.i32(<8 x ptr> %ptrs, i32 %inc, <8 x i1> %mask)
-    declare <vscale x 2 x i64> @llvm.experimental.vector.histogram.add.nxv2p0.i64(<vscale x 2 x ptr> %ptrs, i64 %inc, <vscale x 2 x i1> %mask)
+    declare void @llvm.experimental.vector.histogram.add.v8p0.i32(<8 x ptr> %ptrs, i32 %inc, <8 x i1> %mask)
+    declare void @llvm.experimental.vector.histogram.add.nxv2p0.i64(<vscale x 2 x ptr> %ptrs, i64 %inc, <vscale x 2 x i1> %mask)
 
 Arguments:
 """"""""""