maintainers: Fix broken links to some Github accounts

Change-Id: I89a451fa22d517f9c59dfa0a74f28deb6d750b8f
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

maintainers.rst

@@ -278,3 +278,5 @@ Xilinx platform port
 .. _TonyXie06: https://github.com/TonyXie06
 .. _vwadekar: https://github.com/vwadekar
 .. _Yann-lms: https://github.com/Yann-lms
+.. _grandpaul: https://github.com/grandpaul
+.. _jwerner-chromium: https://github.com/jwerner-chromium