pk: fix a return value and a typo in comment

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 3a5543a..ffd1b73 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -282,7 +282,7 @@
      * - the following fields are used for all public key operations: signature
      *   verify, key pair check and key write.
      * Of course, when MBEDTLS_PK_USE_PSA_EC_DATA is not enabled, the legacy
-     * ecp_keypair structure is used for storing the public key and perform
+     * ecp_keypair structure is used for storing the public key and performing
      * all the operations.
      *
      * Note: This new public key storing solution only works for EC keys, not