Rename Filters to Operators.

I got feedback from several people that they get the wrong idea from the word "Filters" and thus never pull down the menu, missing all the examples inside. "Operators" sits better with April King. Let's see if it does any better. (This is what Google calls them.) We should also add a syntax reference to the help menu.

dxr/templates/layout.html

@@ -42,7 +42,7 @@
                           <button type="button" class="sf-select-trigger" aria-label="Select Filter">
                               <!-- arrow icon using icon font -->
                               <span aria-hidden="true" data-filter-arrow="&#xe801;" class="sf-selector-arrow">
-                                  Filters
+                                  Operators
                               </span>
                           </button>
                         </section>