doc: Revert ALTER TABLESPACE summary line

It was changed when ALTER TABLESPACE / MOVE was added but then not
updated when that was moved back out.

doc/src/sgml/ref/alter_tablespace.sgml

@@ -16,7 +16,7 @@ PostgreSQL documentation
 
  <refnamediv>
   <refname>ALTER TABLESPACE</refname>
-  <refpurpose>change the definition of a tablespace or affect objects of a tablespace</refpurpose>
+  <refpurpose>change the definition of a tablespace</refpurpose>
  </refnamediv>
 
  <refsynopsisdiv>