Fix typo: 'is an error occurred' in documentation

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6229)

doc/man3/OBJ_nid2obj.pod

diff --git a/doc/man3/OBJ_nid2obj.pod b/doc/man3/OBJ_nid2obj.pod
index 6c1aa13..6d85c11 100644
--- a/doc/man3/OBJ_nid2obj.pod
+++ b/doc/man3/OBJ_nid2obj.pod
@@ -54,7 +54,7 @@
 
 OBJ_nid2obj(), OBJ_nid2ln() and OBJ_nid2sn() convert the NID B<n> to
 an ASN1_OBJECT structure, its long name and its short name respectively,
-or B<NULL> is an error occurred.
+or B<NULL> if an error occurred.
 
 OBJ_obj2nid(), OBJ_ln2nid(), OBJ_sn2nid() return the corresponding NID
 for the object B<o>, the long name <ln> or the short name <sn> respectively