Fix a typo in 005_char_signedness.pl test.

The test in 005_char_signedness.pl was missing a dash in the
--set-char-signedness option. Although the test didn't fail since it
doesn't check the error message, it resulted in an unexpected error
message instead of the intended one.

Oversight in 1aab680.

Author: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://postgr.es/m/87tt8h1vb7.fsf@wibble.ilmari.org

Author: MasahikoSawada

src/bin/pg_upgrade/t/005_char_signedness.pl

@@ -51,7 +51,7 @@
 		'-s', $new->host,
 		'-p', $old->port,
 		'-P', $new->port,
-		'-set-char-signedness', 'signed',
+		'--set-char-signedness', 'signed',
 		$mode
 	],
 	'--set-char-signedness option cannot be used for upgrading from v18 or later'