fix: clarify intent of alignment

Author: justjanne

res/css/views/rooms/_EventBubbleTile.scss

@@ -449,8 +449,11 @@ limitations under the License.
 
     .mx_ReadReceiptGroup {
         position: absolute;
-        right: -78px; // as close to right gutter without clipping as possible
-        bottom: -1px;
+        // as close to right gutter without clipping as possible
+        right: -78px;
+        // (EventTileLine.line-height - ReadReceiptGroup.height) / 2
+        // this centers the ReadReceiptGroup if we’ve got a single line
+        bottom: calc(($font-18px - 24px) / 2);
         top: auto;
     }