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Since the EDK2 GenerateCapsule script is out of date and it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: NMasami Hiramatsu <masami.hiramatsu@linaro.org>

doc/board/emulation/qemu_capsule_update.rst

@@ -39,16 +39,9 @@ In addition, the following config needs to be disabled(QEMU ARM specific)::
 
     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