Reword memory terminology for PQresultMemorySize

Rather than using the generic word "space" we might as well use "memory"
since that's precisely what we're dealing with here.

This was extracted from a larger patch around terminology changes where
the remaining hunks were rejected.

Author: Gurjeet Singh <gurjeet@singh.im>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CABwTF4UHO_NtcsOL6_XZfnpKg_0XBFKa7B-7_x5zs3MRZm3-Tg@mail.gmail.com

Author: danielgustafsson

doc/src/sgml/libpq.sgml

@@ -7323,7 +7323,7 @@ size_t PQresultMemorySize(const PGresult *res);
      <para>
       This value is the sum of all <function>malloc</function> requests
       associated with the <structname>PGresult</structname> object, that is,
-      all the space that will be freed by <xref linkend="libpq-PQclear"/>.
+      all the memory that will be freed by <xref linkend="libpq-PQclear"/>.
       This information can be useful for managing memory consumption.
      </para>
     </listitem>