Update docs/getting_started.md

Co-Authored-By: Gilles Peskine <gilles.peskine@arm.com>
diff --git a/docs/getting_started.md b/docs/getting_started.md
index 6b87fa8..b4a2554 100644
--- a/docs/getting_started.md
+++ b/docs/getting_started.md
@@ -557,7 +557,7 @@
 1. Provide `secret` with `psa_key_derivation_input_key()`, referencing a key that
    can be used for key derivation.
 1. Set the key attributes desired for the new derived key. We'll set
-   the `PSA_KEY_USAGE_ENCRYPT` parameter and the `PSA_ALG_CTR` algorithm for this
+   the `PSA_KEY_USAGE_ENCRYPT` usage flag and the `PSA_ALG_CTR` algorithm for this
    example.
 1. Derive the key by calling `psa_key_derivation_output_key()`.
 1. Clean up the key derivation context.