fix inappropriate description for function in RSA

Signed-off-by: Yanray Wang <yanray.wang@arm.com>
diff --git a/library/rsa.c b/library/rsa.c
index fc7f3bc..63102d7 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -500,7 +500,7 @@
 }
 
 /*
- * Get padding mode of RSA modulus
+ * Get padding mode of initialized RSA context
  */
 int mbedtls_rsa_get_padding_mode(const mbedtls_rsa_context *ctx)
 {