doc: qemu: arm64: Fix the documentation of capsule update

Since the EDK2 GenerateCapsule script is out of date and it
doesn't generate the supported version capsule file, the document
should refer the mkeficapsule in tools.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>
diff --git a/doc/board/emulation/qemu_capsule_update.rst b/doc/board/emulation/qemu_capsule_update.rst
index 33ce4bc..0a2286d 100644
--- a/doc/board/emulation/qemu_capsule_update.rst
+++ b/doc/board/emulation/qemu_capsule_update.rst
@@ -39,16 +39,9 @@
 
     CONFIG_TFABOOT
 
-The capsule file can be generated by using the GenerateCapsule.py
-script in EDKII::
-
-    $ ./BaseTools/BinWrappers/PosixLike/GenerateCapsule -e -o \
-    <capsule_file_name> --fw-version <val> --lsv <val> --guid \
-    e2bb9c06-70e9-4b14-97a3-5a7913176e3f --verbose --update-image-index \
-    <val> --verbose <u-boot.bin>
+The capsule file can be generated by using the tools/mkeficapsule::
 
-The above is a wrapper script(GenerateCapsule) which eventually calls
-the actual GenerateCapsule.py script.
+    $ mkeficapsule --raw <u-boot.bin> --index 1 <capsule_file_name>
 
 As per the UEFI specification, the capsule file needs to be placed on
 the EFI System Partition, under the \EFI\UpdateCapsule directory. The