Ensure that the zoom buttons are disabled correctly if the `scale` is smaller/larger than `MIN_SCALE/MAX_SCALE` in `PDFViewerApplication._updateUIToolbar`

In the `zoom{In, Out}` functions in `PDFViewerApplication`, we prevent the zoom value from becoming smaller/larger than `MIN_SCALE/MAX_SCALE`.
However, if the user sets the zoom level through the hash parameter the zoom buttons might not be correctly disabled; try http://mozilla.github.io/pdf.js/web/viewer.html#zoom=10.

Note that this issue has been present since "forever", but given that the solution is so simple I think that we should just fix this. (I'm also fixing a stupid typo I previously made in the JSDoc comment.)

web/app.js

@@ -1211,7 +1211,7 @@ var PDFViewerApplication = {
    * @typedef UpdateUIToolbarParameters
    * @property {number} pageNumber
    * @property {string} scaleValue
-   * @property {scale} scale
+   * @property {number} scale
    * @property {boolean} resetNumPages
    */
 
@@ -1262,8 +1262,8 @@ var PDFViewerApplication = {
     toolbarConfig.firstPage.disabled = (pageNumber <= 1);
     toolbarConfig.lastPage.disabled = (pageNumber >= pagesCount);
 
-    toolbarConfig.zoomOut.disabled = (scale === MIN_SCALE);
-    toolbarConfig.zoomIn.disabled = (scale === MAX_SCALE);
+    toolbarConfig.zoomOut.disabled = (scale <= MIN_SCALE);
+    toolbarConfig.zoomIn.disabled = (scale >= MAX_SCALE);
 
     selectScaleOption(scaleValue, scale);
   },