docs(rme): update tftf build command

Deprecate pack_realm build command for TFTF.
To build Realm payload tests use ENABLE_REALM_PAYLOAD_TESTS=1.
This new command line for TFTF is effective from SHA 9945bef6b
in tf-a-tests repo.

Signed-off-by: Shruti Gupta <shruti.gupta@arm.com>
Change-Id: Iee9ac9b2b367aac50677fac95631e7e4818cdf3a
diff --git a/docs/components/realm-management-extension.rst b/docs/components/realm-management-extension.rst
index 919eea5..f228e6b 100644
--- a/docs/components/realm-management-extension.rst
+++ b/docs/components/realm-management-extension.rst
@@ -157,7 +157,7 @@
 
  git clone https://git.trustedfirmware.org/TF-A/tf-a-tests.git
  cd tf-a-tests
- make CROSS_COMPILE=aarch64-none-elf- PLAT=fvp DEBUG=1 all pack_realm
+ make CROSS_COMPILE=aarch64-none-elf- PLAT=fvp DEBUG=1 ENABLE_REALM_PAYLOAD_TESTS=1 all
 
 This produces a TF-A Tests binary (**tftf.bin**) with Realm payload packaged
 and **sp_layout.json** in the **build/fvp/debug** directory.