doc: Fix missing whitespace in pg_restore documentation.

doc: Fix missing whitespace in pg_restore documentation.

Previously, a space was missing between "<option>--exclude-schema</option>"
and "for" in the pg_restore documentation. This commit fixes the typo by
adding the missing whitespace.

Back-patch to v17 where the typo was added.

Author: Lele Gaifax <lele@metapensiero.it>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/87lds3ysm0.fsf@metapensiero.it
Backpatch-through: 17