Fix typo in documentation of MBEDTLS_SSL_CONF_RNG in config.h
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 96413f8..69601de 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -3666,7 +3666,7 @@
//#define MBEDTLS_SSL_CONF_RECV_TIMEOUT mbedtls_net_recv_timeout
/* The PRNG to use by the SSL module. If defined, this must
- * evaluate to the name on externally defined function with signature
+ * evaluate to the name of an externally defined function with signature
* int f_rng(void *, unsigned char *, size_t),
* which ignores its first parameter (the stack will always
* pass NULL to this function).