Fix typo 'is an error occurred' in documentation

Once all signer certificates have been set CMS_verify() can be used.

Although CMS_get0_SignerInfos() can return NULL is an error occur B<or> if

Although CMS_get0_SignerInfos() can return NULL if an error occurs B<or> if

there are no signers this is not a problem in practice because the only

error which can occur is if the B<cms> structure is not of type signedData

due to application error.

CMS_ReceiptRequest_create0() returns a signed receipt request structure or

NULL if an error occurred.

CMS_add1_ReceiptRequest() returns 1 for success or 0 is an error occurred.

CMS_add1_ReceiptRequest() returns 1 for success or 0 if an error occurred.

CMS_get1_ReceiptRequest() returns 1 is a signed receipt request is found and

decoded. It returns 0 if a signed receipt request is not present and -1 if

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

=head1 RETURN VALUES

SMIME_read_PKCS7() returns a valid B<PKCS7> structure or B<NULL>

is an error occurred. The error can be obtained from ERR_get_error(3).

if an error occurred. The error can be obtained from ERR_get_error(3).

=head1 SEE ALSO