show-doc raises error when called on a Struct field method

```
[4] pry(main)> S = Struct.new(:foo)
=> S
[5] pry(main)> ? S#foo
NameError: undefined method `new' for class `S'
from /home/gutov/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/pry-doc-0.8.0/lib/pry-doc.rb:63:in `instance_method'
```
