doc: fix wrong tag used in create sequence manual.

In ref/create_sequence.sgml <literal> tag was used for nextval function name.
This should have been <function> tag.

Author: Noboru Saito
Discussion: https://postgr.es/m/CAAM3qnJTDFFfRf5JHJ4AYrNcqXgMmj0pbH0%2Bvm%3DYva%2BpJyGymA%40mail.gmail.com
Backpatch-through: 10

doc/src/sgml/ref/create_sequence.sgml

@@ -288,7 +288,7 @@ SELECT * FROM <replaceable>name</replaceable>;
    session A might reserve values 1..10 and return
    <function>nextval</function>=1, then session B might reserve values
    11..20 and return <function>nextval</function>=11 before session A
-   has generated <literal>nextval</literal>=2.  Thus, with a
+   has generated <function>nextval</function>=2.  Thus, with a
    <replaceable class="parameter">cache</replaceable> setting of one
    it is safe to assume that <function>nextval</function> values are generated
    sequentially; with a <replaceable