docs: update MAINTAINERS.md with pbertho

Add Pierre Bertholom to MAINTAINERS.md

Signed-off-by: PierreBertholom <pbertho@amazon.com>

Author: PierreBertholom

MAINTAINERS.md

@@ -9,5 +9,6 @@ Firecracker is maintained by a dedicated team within Amazon:
 - Jay Chung <jaehoc@amazon.com>
 - Marco Marangoni <mamarang@amazon.com>
 - Michael Zoumboulakis <zoumboul@amazon.com>
+- Pierre Bertholom <pbertho@amazon.com>
 - Riccardo Mancini <mancio@amazon.com>
 - Takahiro Itazuri <itazur@amazon.com>