feat: patch for std4#219 (#6622)

Co-authored-by: Scott Morrison <scott.morrison@gmail.com>

Author: fgdorais

Mathlib/Tactic/ToAdditive.lean

@@ -96,7 +96,7 @@ Id.run do
   if (s.get i₁).isUpper then
     if let some strs := endCapitalNames.find? (s.extract 0 i₁) then
       if let some (pref, newS) := strs.findSome?
-        fun x ↦ (s.extract i₁ s.endPos).dropPrefix? x |>.map (x, ·.toString) then
+        fun x : String ↦ (s.extract i₁ s.endPos).dropPrefix? x |>.map (x, ·.toString) then
         return splitCase newS 0 <| (s.extract 0 i₁ ++ pref)::r
     if !(s.get i₀).isUpper then
       return splitCase (s.extract i₁ s.endPos) 0 <| (s.extract 0 i₁)::r