Update rsa.h

minor change to the file's brief desc.
diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index 033e580..f8b8965 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -1,7 +1,7 @@
 /**
  * \file rsa.h
  *
- * \brief This file defines the RSA public-key cryptosystem.
+ * \brief This file provides an API for the RSA public-key cryptosystem.
  *
  * The RSA public-key cryptosystem is defined in <em>Public-Key
  * Cryptography Standards (PKCS) #1 v1.5: RSA Encryption</em>