Fixes typo
Commit fixes typo in rsa.h found in review.
@@ -570,7 +570,7 @@ int mbedtls_rsa_private( mbedtls_rsa_context *ctx,
* operation.
*
* \param ctx The initialized RSA context to use.
- * \param f_rng The RNG to use. It used for padding generation
+ * \param f_rng The RNG to use. It is used for padding generation
* and it is mandatory.
* \param p_rng The RNG context to be passed to \p f_rng. May be
* \c NULL if \p f_rng doesn't need a context argument.
