doc: fix wording for plpgsql, add "and"

Reported-by: Anthony Greene

Discussion: https://postgr.es/m/CAPRNmnsSZ4QL75FUjcS8ND_oV+WjgyPbZ4ch2RUwmW6PWzF38w@mail.gmail.com

Backpatch-through: 9.4

doc/src/sgml/plpgsql.sgml

@@ -5193,7 +5193,7 @@ HINT:  Make sure the query returns the exact list of columns.
   <para>
    <application>PL/pgSQL</application> is similar to PL/SQL in many
    aspects. It is a block-structured, imperative language, and all
-   variables have to be declared.  Assignments, loops, conditionals
+   variables have to be declared.  Assignments, loops, and conditionals
    are similar.  The main differences you should keep in mind when
    porting from <application>PL/SQL</application> to
    <application>PL/pgSQL</application> are: