Doc: fix incorrectly stated argument list for pgcrypto's hmac() function.

The bytea variant takes (bytea, bytea, text).
Per unsigned report.

Discussion: https://postgr.es/m/153344327294.1404.654155870612982042@wrigleys.postgresql.org

doc/src/sgml/pgcrypto.sgml

@@ -63,7 +63,7 @@ $$ LANGUAGE SQL STRICT IMMUTABLE;
 
 <synopsis>
 hmac(data text, key text, type text) returns bytea
-hmac(data bytea, key text, type text) returns bytea
+hmac(data bytea, key bytea, type text) returns bytea
 </synopsis>
 
    <para>