Adjust wording for update icon tooltip

As per discussion at
https://hypothes-is.slack.com/archives/frontend/p1474276933000039,
adjust the wording of the 'Apply Update' icon tooltip to avoid "update"
being misinterpreted as referring to a software update.

h/templates/client/top_bar.html

@@ -31,7 +31,7 @@
        ng-click="onApplyPendingUpdates()"
        h-tooltip
        tooltip-direction="up"
-       aria-label="New updates available">
+       aria-label="Show {{pendingUpdateCount}} new/updated annotation(s)">
        <svg-icon class="top-bar__apply-icon" name="'refresh'"></svg-icon>
     </a>
     <search-input