Change wording in perf summary PR comment to expose the used aggregation statistic

Author: Kobzol

site/src/comparison.rs

@@ -376,15 +376,15 @@ impl ComparisonSummary {
         if self.num_regressions > 1 {
             writeln!(
                 result,
-                "- Average relevant regression: {:.1}%",
+                "- Arithmetic mean of relevant regression(s): {:.1}%",
                 self.average_regression()
             )
             .unwrap();
         }
         if self.num_improvements > 1 {
             writeln!(
                 result,
-                "- Average relevant improvement: {:.1}%",
+                "- Arithmetic mean of relevant improvements(s): {:.1}%",
                 self.average_improvement()
             )
             .unwrap();