demo: Fix makefile target

The makefile target was incorrect and wouldn't build the rsa_encrypt demo.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20503)

(cherry picked from commit 67bfdfa17bc4ca8e2b819316299bb82748394c45)

demos/encrypt/Makefile

@@ -12,7 +12,7 @@ all: rsa_encrypt
 %.o: %.c
 	$(CC) $(CFLAGS) -c $<
 
-rsa_encrypt_ec: rsa_encrypt.o
+rsa_encrypt: rsa_encrypt.o
 
 test: ;