Fix incorrect gutter bail message

[t-taylor]

"spacer" was listed twice in the editors.gutter example and the error message for the gutter config parsing was missing newer options.

[pascalkuthe]

Please run rustfmt to make CI pass

[the-mikedavis]

The change in the bail! message looks good but I believe the configuration docs are correct:

helix/helix-view/src/editor.rs

Lines 86 to 99 in 57e538d

	impl Default for GutterConfig {
	fn default() -> Self {
	Self {
	layout: vec![
	GutterType::Diagnostics,
	GutterType::Spacer,
	GutterType::LineNumbers,
	GutterType::Spacer,
	GutterType::Diff,
	],
	line_numbers: GutterLineNumbersConfig::default(),
	}
	}
	}

Gutter elements are repeatable and spacer is used to add some space between other elements.

[pascalkuthe]

ah right I misserad the change. The docs show the default configureitaton (which contains two spacers) not the list of available options