[FIX] doc: update links

Before this commit, Installation Steps and developer tutorials were
referenced the master version.  In this commit, the references are
corrected to the current version.

closes odoo/odoo#47142

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

README.md

@@ -27,7 +27,7 @@ a full-featured <a href="https://www.odoo.com">Open Source ERP</a> when you inst
 
 Getting started with Odoo
 -------------------------
-For a standard installation please follow the <a href="https://www.odoo.com/documentation/master/setup/install.html">Setup instructions</a>
+For a standard installation please follow the <a href="https://www.odoo.com/documentation/12.0/setup/install.html">Setup instructions</a>
 from the documentation.
 
-Then follow <a href="https://www.odoo.com/documentation/master/tutorials.html">the developer tutorials</a>
+Then follow <a href="https://www.odoo.com/documentation/12.0/tutorials.html">the developer tutorials</a>