Doc: fix prompt in psql crosstabview example
The prompt in the crosstabview example was incorrectly indicating
an open parenthesis where there is none in the example query. Fix
by changing to the normal multi-line prompt.
Reported-by: y.saburov@gmail.com
Discussion: https://postgr.es/m/
171369707837.684.
8207966689229976474@wrigleys.postgresql.org