Doc: fix copy-and-pasteo in ecpg docs.

The synopsis for PGTYPESinterval_free() used the wrong name.

Discussion: https://postgr.es/m/159231203030.679.3061023914894071953@wrigleys.postgresql.org

doc/src/sgml/ecpg.sgml

@@ -3372,7 +3372,7 @@ interval *PGTYPESinterval_new(void);
        <para>
         Release the memory of a previously allocated interval variable.
 <synopsis>
-void PGTYPESinterval_new(interval *intvl);
+void PGTYPESinterval_free(interval *intvl);
 </synopsis>
        </para>
       </listitem>