chore: "fix" <|> notation declaration

The `infix` declaration was generating a delaborator, but it is
producing the invalid term
```
ConstantFunction.f myFun 3 <|> fun _ => ConstantFunction.f myFun 4
```
It is invalid because the `macro_rules` for `<|>` is based on
`binop_lazy%` which introduces the `fun _ =>` for us.

I tried to use
```
notation:20 a:21 " <|> " b:20 => HOrElse.hOrElse a fun _ : Unit => b
```
but the delaborator generator does not work for this case.

src/Init/Notation.lean

@@ -128,7 +128,7 @@ infixl:30 " || " => or
 notation:max "!" b:40 => not b
 
 infixr:67 " :: " => List.cons
-infixr:20 " <|> " => HOrElse.hOrElse
+syntax:20 term:21 " <|> " term:20 : term
 infixr:60  " >> "  => HAndThen.hAndThen
 infixl:55  " >>= " => Bind.bind
 infixl:60  " <*> " => Seq.seq

tests/lean/eagerCoeExpansion.lean.expected.out

@@ -22,4 +22,4 @@ fun (a : Nat) =>
         (@OfNat.ofNat.{0} Nat 2 (instOfNatNat 2)))
       true)
 def s : Option Nat :=
-ConstantFunction.f myFun 3 <|> ConstantFunction.f myFun 4
+HOrElse.hOrElse (ConstantFunction.f myFun 3) fun x => ConstantFunction.f myFun 4