doc: Fix native reStructuredText example for tags, surrounding them with brackets

Here is yet another small fix in doc, unifomizing how tags must be set in reStructuredText example entry with others (YAML, markdown).

As a side note, it could be cleaner IMHO if in native metadata style (markdown, reST) those brackets could be ignored. I tried but failed to quicly find where parsing tags occurs, to measure feasability of such a change. What do you think about ?-)