Fix nomethod error

Every time I run ri, I get a NoMethodError. Below is the last line of the backtrace:
```
/Users/gjaldon/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rdoc-3.12.2/lib/rdoc/markup/attribute_manager.rb:180:in `mask_protected_sequences': undefined method `gsub!' for ["read_array"]:Array (NoMethodError)
```
The method expects `@str` to be a String so just converted @str with String as a quick fix for it to work on my machine. Might be helpful to minimize the chances of others to experience this same bug. Couldn't find the test for it though. 

Let me know if this helps. Cheers!