ssl.h: spelling in comment

Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1275)

include/openssl/ssl.h

@@ -1277,7 +1277,7 @@ DECLARE_PEM_rw(SSL_SESSION, SSL_SESSION)
 # define SSL_need_tmp_RSA(ssl)                    0
 # define SSL_set_tmp_rsa(ssl,rsa)                 1
 /*
- * We "preted" to call the callback to avoid warnings about unused static
+ * We "pretend" to call the callback to avoid warnings about unused static
  * functions.
  */
 # define SSL_CTX_set_tmp_rsa_callback(ctx, cb)    while(0) (cb)(NULL, 0, 0)