doc: Correct text about gpgme_cancel_async.
doc: Correct text about gpgme_cancel_async.

* doc/gpgme.texi: Documentation about gpgme_cancel_async should refer
to the correct name.

Signed-off-by: Daniel Kahn Gillmor <b1356865a0eefd1fb76a8b663fad027be4bc9b61@fifthhorseman.net>

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme