Fix result.rule.description in the schema

In the SARIF schema, result.ruleId.description is

The stable, unique identifier of the rule, if any, to which this notification is relevant. This member can be used to retrieve rule metadata from the rules dictionary, if it exists.

• In the first sentence, it should say "to which this result is relevant".
• The second sentence should be removed. The spec is clear that rule lookup is done by ruleIndex, not by ruleId.