Typo fixes for previous commit.

Noted by Thom Brown.

doc/src/sgml/datatype.sgml

@@ -832,9 +832,9 @@ ALTER SEQUENCE <replaceable class="parameter">tablename</replaceable>_<replaceab
     <note>
       <para>
         Because <type>smallserial</type>, <type>serial</type> and
-        <type>bigserial</type> are implemented usings sequences, there may
+        <type>bigserial</type> are implemented using sequences, there may
         be "holes" or gaps in the sequence of values which appears in the
-        column, even if no rows are ever deleted.  This is a value allocated
+        column, even if no rows are ever deleted.  A value allocated
         from the sequence is still "used up" even if a row containing that
         value is never successfully inserted into the table column.  This
         may happen, for example, if the inserting transaction rolls back.