Fix comment in fe-auth-scram.c

The frontend-side routine in charge of building a SCRAM verifier
mentioned that the restrictions applying to SASLprep on the password
with the encoding are described at the top of fe-auth-scram.c, but this
information is in auth-scram.c.

This is wrong since 8f8b9be, so backpatch all the way down as this is an
important documentation bit.

Spotted while reviewing a different patch.

Backpatch-through: 11

src/interfaces/libpq/fe-auth-scram.c

@@ -774,7 +774,8 @@ pg_fe_scram_build_verifier(const char *password)
 	/*
 	 * Normalize the password with SASLprep.  If that doesn't work, because
 	 * the password isn't valid UTF-8 or contains prohibited characters, just
-	 * proceed with the original password.  (See comments at top of file.)
+	 * proceed with the original password.  (See comments at the top of
+	 * auth-scram.c.)
 	 */
 	rc = pg_saslprep(password, &prep_password);
 	if (rc == SASLPREP_OOM)