Update ErrorType.swift documentation on IntParsingError (#16092)

Update ErrorType.swift

Example on IntParsingError should have Character as associated value type not String.
Also updated sample code to use c rather than s

Author: kandelvijaya

stdlib/public/core/ErrorType.swift

@@ -35,7 +35,7 @@ import SwiftShims
 ///
 ///     enum IntParsingError: Error {
 ///         case overflow
-///         case invalidInput(String)
+///         case invalidInput(Character)
 ///     }
 ///
 /// The `invalidInput` case includes the invalid character as an associated
@@ -48,8 +48,9 @@ import SwiftShims
 ///     extension Int {
 ///         init(validating input: String) throws {
 ///             // ...
-///             if !_isValid(s) {
-///                 throw IntParsingError.invalidInput(s)
+///             let c = _nextCharacter(from: input)
+///             if !_isValid(c) {
+///                 throw IntParsingError.invalidInput(c)
 ///             }
 ///             // ...
 ///         }