docs(guide/expression): replace tt by code

Replaces <tt> elements with <code> in expressions guide. Looks identical
in Chromium

Closes angular#12437

Conflicts:
	docs/content/guide/expression.ngdoc

Author: Laisky

docs/content/guide/expression.ngdoc

@@ -70,7 +70,7 @@ You can try evaluating different expressions here:
       <ul>
        <li ng-repeat="expr in exprs track by $index">
          [ <a href="" ng-click="removeExp($index)">X</a> ]
-         <tt>{{expr}}</tt> => <span ng-bind="$parent.$eval(expr)"></span>
+         <code>{{expr}}</code> => <span ng-bind="$parent.$eval(expr)"></span>
         </li>
       </ul>
     </div>
@@ -344,5 +344,4 @@ When using a directive that takes an expression:
 <ul>
   <li ng-repeat="item in ::items">{{item.name}};</li>
 </ul>
-```
-
+```