Docs: SPRTL: Wording update for 'local storage'

Using phrase 'local storage' to replace 'scratch' and 'metadata'
and rename the file to remove the redundant namespace.

Signed-off-by: Ken Liu <Ken.Liu@arm.com>
Change-Id: I84bc9762e0da0aefee83039623cded05761f1d7d
diff --git a/docs/design_docs/ff_isolation.rst b/docs/design_docs/ff_isolation.rst
index ad0000e0..e999cbc 100644
--- a/docs/design_docs/ff_isolation.rst
+++ b/docs/design_docs/ff_isolation.rst
@@ -175,7 +175,7 @@
 There is re-usable code like the C-Runtime and RoT Service API which are same
 across different partitions. TF-M creates a Secure Partition Runtime Library
 (SPRTL) as a specific library shared by the Secure Partition. Please refer to
-:doc:`Secure Partition Runtime Library </design_docs/services/tfm_secure_partition_runtime_library>`
+:doc:`Secure Partition Runtime Library </design_docs/services/secure_partition_runtime_library>`
 for more detail.
 
 .. note::