Fix minor grammatical error
Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/docs/architecture/psa-migration/psa-limitations.md b/docs/architecture/psa-migration/psa-limitations.md
index 724e29b..f5b5700 100644
--- a/docs/architecture/psa-migration/psa-limitations.md
+++ b/docs/architecture/psa-migration/psa-limitations.md
@@ -317,7 +317,7 @@
HKDF: Expand not exposed on its own (TLS 1.3)
---------------------------------------------
-The HKDF function uses and Extract-then-Expand approach, that is:
+The HKDF function uses an Extract-then-Expand approach, that is:
HKDF(x, ...) = HKDF-Expand(HKDF-Extract(x, ...), ...)