modify links in readme to the tutorials (#644)

Signed-off-by: Coline PILOQUET <coline.piloquet@rte-france.com>

README.md

@@ -58,7 +58,7 @@ The main features are:
 
 ## Getting started with PowSyBl diagram
 
-- Have a look at the tutorials presented [here](https://www.powsybl.org/pages/documentation/developer/tutorials/) and learn [how to customize your single-line diagram](https://www.powsybl.org/pages/documentation/developer/tutorials/diagram/sld-custom-node.html).
+- Have a look at the tutorials presented [here](https://powsybl.readthedocs.io/projects/powsybl-tutorials/en/latest/) and learn [how to customize your single-line diagram](https://powsybl.readthedocs.io/projects/powsybl-tutorials/en/latest/diagram/sld-custom-node.html).
 
 - Pop in the [SLD](https://powsybl.readthedocs.io/projects/powsybl-diagram/latest/single_line_diagrams/index.html) and [NAD](https://powsybl.readthedocs.io/projects/powsybl-diagram/latest/network_area_diagrams/index.html) documentation pages to get some examples on how to use PowSyBl diagrams!