data-pprint/Data/Data/GenRep/Doc.hs: don't rely on IsString instance for Doc

Early versions of the "pretty" library don't provide an IsString instance for
Doc. This causes the build of this package to fail with older compilers. We
work around this issue by calling 'text' explicitly where it is required.
Merge pull request #3 from peti/master

data-pprint/Data/Data/GenRep/Doc.hs: don't rely on IsString instance for Doc