rustfmt: Use show_parse_errors

There are no behaviour changes with this patch applied.

In rust-lang/rustfmt#5961 the
`hide_parse_errors` was changed to `show_parse_errors` with default
`true`.

Since we run the formatter with nightly we are getting the following
warning

 Warning: the `hide_parse_errors` option is deprecated. Use
 `show_parse_errors` instead

Rename the variable and keep the default value `true`.

Author: tcharding

rustfmt.toml

@@ -70,7 +70,7 @@ color = "Auto"
 unstable_features = false
 disable_all_formatting = false
 skip_children = false
-hide_parse_errors = false
+show_parse_errors = true
 error_on_line_overflow = false
 error_on_unformatted = false
 emit_mode = "Files"