docs: change all occurrences of RSS to RSE

Changes all occurrences of "RSS" and "rss" in the documentation
to "RSE" and "rse".

Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Change-Id: Ia42078f5faa1db331b1e5a35f01faeaf1afacb5f
diff --git a/docs/design_documents/index.rst b/docs/design_documents/index.rst
index ecc68b2..ac982e0 100644
--- a/docs/design_documents/index.rst
+++ b/docs/design_documents/index.rst
@@ -9,7 +9,7 @@
    context_mgmt_rework
    measured_boot_poc
    drtm_poc
-   rss
+   rse
    psci_osi_mode
    measured_boot