Small improvement to serialize docs

[FlorianLudwig]

Docs state that serialize returns a string but returns bytes.

[coveralls]

Coverage decreased (-0.06%) to 75.506% when pulling c589e76 on FlorianLudwig:serialize-doc into b3bb4db on RDFLib:master.

[coveralls]

Coverage increased (+0.002%) to 75.565% when pulling c589e76 on FlorianLudwig:serialize-doc into b3bb4db on RDFLib:master.

[ashleysommer]

Thanks. This is a hangover from the python2 days when it did return str, back when str was byte-encoded.
Ever since RDFLib 4.0, serialize() returns bytes, this was never updated in the documentation.

Note, we are planning a change to serialize() in RDFLib 6.0, so it does return a unicode string.
But I will merge this for now anyway.