Fix indent in code examle of Object#{to_enum|enum_for}
Object#{to_enum|enum_for} does not raise exceptions

When calling a generated enumerator, the enumerator raises a NoMethodError.

    enum = 1.enum_for(:not_defined)
    enum.next #=> NoMethodError: undefined method `not_defined' for 1:Fixnum
Merge pull request #10 from kachick/fix-to_enum-exception

Remove exceptions from Object#{to_enum|enum_for}