Simplify syntax diagram for REINDEX.

Simplify syntax diagram for REINDEX.

Since there currently is only one possible parenthesized option, namely
VERBOSE, it's a bit pointless to show it with "{ } [, ... ]".  The curly
braces are useless and therefore confusing, as seen in a recent question
from Karsten Hilbert.  Remove the extra decoration for the time being;
we can put it back when and if REINDEX grows some more options.