Fix bn_wexpand's documentation.

Change-Id: I15e243c9732f2d3ab77b2fa77e0c3c10d5c7f3af
Reviewed-on: https://boringssl-review.googlesource.com/10541
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/bn.h

@@ -327,7 +327,7 @@ OPENSSL_EXPORT int BN_marshal_asn1(CBB *cbb, const BIGNUM *bn);
 OPENSSL_EXPORT void bn_correct_top(BIGNUM *bn);
 
 /* bn_wexpand ensures that |bn| has at least |words| works of space without
- * altering its value. It returns one on success or zero on allocation
+ * altering its value. It returns |bn| on success or NULL on allocation
  * failure. */
 OPENSSL_EXPORT BIGNUM *bn_wexpand(BIGNUM *bn, size_t words);