Docs: Fix spelling typos

Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
Change-Id: Ie7729b848b7284696d9b546b676cb4f31ab5db47
diff --git a/docs/design_docs/ff_isolation.rst b/docs/design_docs/ff_isolation.rst
index b9727cb..f078dc2 100644
--- a/docs/design_docs/ff_isolation.rst
+++ b/docs/design_docs/ff_isolation.rst
@@ -176,7 +176,7 @@
 one running at a time and all others are deactivated, the SPM needs to set up
 necessary regions for each asset type in one partition only.
 
-There is re-usable code like the C-Runtime and RoT Service API which are same
+There is reusable 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/secure_partition_runtime_library>`