Docs: Correct some spelling error

Change-Id: I0a7006dc816b204284bce2793187942250720a18
Signed-off-by: Edison Ai <edison.ai@arm.com>
diff --git a/docs/design_documents/tfm_its_512_flash.rst b/docs/design_documents/tfm_its_512_flash.rst
index 6ce144d..aa58c2b 100644
--- a/docs/design_documents/tfm_its_512_flash.rst
+++ b/docs/design_documents/tfm_its_512_flash.rst
@@ -12,7 +12,7 @@
 
 The proposal is describing a mechanism to enable the use of larger flash
 devices, imposing a requirement for word-aligned full-block program operations,
-in Trusted Firmare-M.
+in Trusted Firmware-M.
 
 
 Requirements