Fix BN_sub documentation.

BUG=405370

Change-Id: Ia44cbdcc4d0d90e610d1a1f2eb49b056b07ae5e4
Reviewed-on: https://boringssl-review.googlesource.com/1660
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/bn.h

@@ -351,11 +351,11 @@ OPENSSL_EXPORT int BN_uadd(BIGNUM *r, const BIGNUM *a, const BIGNUM *b);
 /* BN_add_word adds |w| to |a|. It returns one on success and zero otherwise. */
 OPENSSL_EXPORT int BN_add_word(BIGNUM *a, BN_ULONG w);
 
-/* BN_sub sets |r| = |a| + |b|, where |r| must be a distinct pointer from |a|
+/* BN_sub sets |r| = |a| - |b|, where |r| must be a distinct pointer from |a|
  * and |b|. It returns one on success and zero on allocation failure. */
 OPENSSL_EXPORT int BN_sub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b);
 
-/* BN_usub sets |r| = |a| + |b|, where |a| and |b| are non-negative integers,
+/* BN_usub sets |r| = |a| - |b|, where |a| and |b| are non-negative integers,
  * |b| < |a| and |r| must be a distinct pointer from |a| and |b|. It returns
  * one on success and zero on allocation failure. */
 OPENSSL_EXPORT int BN_usub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b);