Rename NID_x25519 to NID_X25519.

I went with NID_x25519 to match NID_sha1 and friends in being lowercase.
However, upstream seems to have since chosen NID_X25519. Match their
name.

Change-Id: Icc7b183a2e2dfbe42c88e08e538fcbd242478ac3
Reviewed-on: https://boringssl-review.googlesource.com/7331
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/crypto/obj/obj_mac.num b/crypto/obj/obj_mac.num
index a0e09b8..074657a 100644
--- a/crypto/obj/obj_mac.num
+++ b/crypto/obj/obj_mac.num
@@ -945,4 +945,4 @@
 dhSinglePass_cofactorDH_sha512kdf_scheme		945
 dh_std_kdf		946
 dh_cofactor_kdf		947
-x25519		948
+X25519		948