Correct the spelling of "primitive".

Spotted by Matt Smart.

Change-Id: Id9c61ba6a293ddc52b2e2c93c427860765848c6d
Reviewed-on: https://boringssl-review.googlesource.com/6430
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/aead.h b/include/openssl/aead.h
index f719f02..aa2b080 100644
--- a/include/openssl/aead.h
+++ b/include/openssl/aead.h
@@ -24,7 +24,7 @@
 
 /* Authenticated Encryption with Additional Data.
  *
- * AEAD couples confidentiality and integrity in a single primtive. AEAD
+ * AEAD couples confidentiality and integrity in a single primitive. AEAD
  * algorithms take a key and then can seal and open individual messages. Each
  * message has a unique, per-message nonce and, optionally, additional data
  * which is authenticated but not included in the ciphertext.