Fix ruby syntax error

Occured this error.
I think it's ruby syntax error.

```
spree/core/app/models/spree/return_item.rb:57: syntax error, unexpected tLPAREN_ARG, expecting keyword_do_LAMBDA or tLAMBEG (SyntaxError)
...epted, from: :pending, if: -> (return_item) { return_item.el...
```