doc: Fix syntax in ALTER FOREIGN DATA WRAPPER example

doc: Fix syntax in ALTER FOREIGN DATA WRAPPER example

The example for dropping an option was incorrectly quoting the
option key thus making it a value turning the command into an
unqualified ADD operation. The result of dropping became adding
a new key/value pair instead:

 d=# alter foreign data wrapper f options (drop 'b');
 ALTER FOREIGN DATA WRAPPER
 d=# select fdwoptions from pg_foreign_data_wrapper where fdwname='f';
  fdwoptions
 ------------
  {drop=b}
 (1 row)

This has been incorrect for a long time so backpatch to all
supported branches.

Author: Tim <tim.needham2@gmail.com>
Discussion: https://postgr.es/m/170292280173.1876505.5204623074024041738@wrigleys.postgresql.org