Avoid slicing a string in DocParsingError

[fmassot]

Done here: https://github.com/quickwit-oss/tantivy/blob/main/src/schema/schema.rs#L402-L412

Better with invalid_json.chars().take(20) :)

[waywardmonkeys]

The version for those line numbers would be

tantivy/src/schema/schema.rs

Lines 402 to 412 in bb5254d

	impl DocParsingError {
	/// Builds a NotJson DocParsingError
	fn invalid_json(invalid_json: &str) -> Self {
	let sample_json: String = if invalid_json.len() < 20 {
	invalid_json.to_string()
	} else {
	format!("{:?}...", &invalid_json[0..20])
	};
	DocParsingError::InvalidJson(sample_json)
	}
	}