Correct U-Boot upstream repository

The U-Boot source moves to https://source.denx.de/u-boot/u-boot.git
effective 2021-02-28.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/build/source.rst b/doc/build/source.rst
index 75720e2..470f793 100644
--- a/doc/build/source.rst
+++ b/doc/build/source.rst
@@ -7,7 +7,7 @@
 
 .. code-block:: bash
 
-    git clone https://gitlab.denx.de/u-boot/u-boot.git
+    git clone https://source.denx.de/u-boot/u-boot.git
 
 A mirror of the source is maintained on Github