Fix isdeprecated() call in doc/DocCheck.jl

Error is
```julia
julia> DocCheck.undocumented_rst()
Loading help data...
ERROR: no method isdeprecated(ASCIIString,Symbol)
 in _undocumented_rst at /home/kmsquire/Source/julia/doc/DocCheck.jl:107
 in undocumented_rst at /home/kmsquire/Source/julia/doc/DocCheck.jl:138
```

I'm not quite sure how this worked before, but either the first parameter needs to be a Module, or needs to be removed.  This pull request jumps through hoops to make it a module. Let me know if there's a better way to do this.