Fix example for bookmap bookowner

[Radu Coravu <radu_coravu@sync.ro>]

Hi,

We got feedback from one of our end users that in the bookmap specs:

https://docs.oasis-open.org/dita/dita/v1.3/os/part2-tech-content/langRef/technicalContent/bookmap.html

there is an example which contains something like:

>      <bookowner>OASIS</bookowner>

but the <bookowner> cannot contain directly text content.
So maybe the example could be fixed.

Regards,
Radu

Radu Coravu
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