Update sparql.md

Two things:

1/ Escaping the <> properly in the GRAPH example

2/ Adding an essential extra about multiple values passed down with the samen request-parameter identifier.  Since this is supported in the code at https://github.com/tdt/core/blob/master/app/controllers/tdt/core/datacontrollers/SPARQLController.php#L272
Merge pull request #25 from marc-portier/patch-1

Update sparql.md