Align spec language in ValidateDefaultValue

[glen-84]

ValidateDefaultValue(defaultValue):

• If {defaultValue} is a ListValue:
  • For each {valueNode} in {defaultValue}:
    • If {ValidateDefaultValue(valueNode)} is false
      • return false
• If {defaultValue} is an ObjectValue:
  • Let {objectFields} be a list of all fields of {defaultValue}
  • Let {fields} be a list of all fields {objectFields} are referring to
  • For each {field} in {fields}:
    • If {IsExposed(field)} is false
      • return false
  • For each {objectField} in {objectFields}:
    • Let {value} be the value of {objectField}
    • return {ValidateDefaultValue(value)}
• If {defaultValue} is an EnumValue:
  • If {IsExposed(defaultValue)} is false
    • return false
• return true

Align objectFields/fields language with https://spec.graphql.org/draft/#sec-Input-Object-Required-Fields.