Doc: replace unnecessary non-breaking space with ordinal space.

There were unnecessary non-breaking spaces (nbsp, U+00A0, 0xc2a0 in
UTF-8) in the docs.  This commit replaces them with ASCII spaces
(0x20).

config.sgml is backpatched through 17.
ref/drop_extension.sgml is backpatched through 13.

Discussion: https://postgr.es/m/20240930.153404.202479334310259810.ishii%40postgresql.org
Reviewed-by: Yugo Nagata, Daniel Gustafsson
Backpatch-through: 17, 13

doc/src/sgml/ref/drop_extension.sgml

@@ -82,7 +82,7 @@ DROP EXTENSION [ IF EXISTS ] <replaceable class="parameter">name</replaceable> [
      <para>
       This option prevents the specified extensions from being dropped if
       other objects, besides these extensions, their members, and their
-      explicitly dependent routines, depend on them.  This is the default.
+      explicitly dependent routines, depend on them. This is the default.
      </para>
     </listitem>
    </varlistentry>