Change the word 'box' to rectangular prism

Summary: For https://trello.com/c/xXg8V01k/351-volume-exercises

Reviewers: eater, emily

Reviewed By: emily

Differential Revision: https://phabricator.khanacademy.org/D15006

exercises/volume_1.html

@@ -98,7 +98,7 @@
             </div>
 
             <p class="question">
-                What is the volume of this box? Drag on the box to rotate it.
+                What is the volume of this rectangular prism? Drag on the rectangular prism to rotate it.
             </p>
 
             <p class="render-answer-area-here"></p>
@@ -208,7 +208,7 @@
                 <p>The volume of a shape is measured by the number of one <var>UNIT</var> cubes which make up the shape.</p>
                 <div>
                     <p>
-                    To make counting the cubes easier, break the box into <code><var>HEIGHT</var></code> slabs which are each
+                    To make counting the cubes easier, break the rectangular prism into <code><var>HEIGHT</var></code> slabs which are each
                     <code><var>WIDTH</var> <var>UNIT_TEXT</var></code> wide,  <code><var>LENGTH</var> <var>UNIT_TEXT</var></code> long, and
                     <code>1 <var>UNIT_TEXT</var></code> high.
                     </p>
@@ -263,9 +263,9 @@
                     </div>
                 </div>
                 <p>Since there are <code><var>WIDTH*LENGTH</var></code> cubes in each slab, there are a total of
-                <code><var>HEIGHT</var> \times <var>WIDTH*LENGTH</var> = <var>HEIGHT*WIDTH*LENGTH</var></code> cubes in the whole box.</p>
+                <code><var>HEIGHT</var> \times <var>WIDTH*LENGTH</var> = <var>HEIGHT*WIDTH*LENGTH</var></code> cubes in the whole rectangular prism.</p>
                 <p>
-                    Since each cube is a cubic <var>UNIT</var>, the volume of the box is <code><var>HEIGHT*WIDTH*LENGTH</var> <var>UNIT_TEXT</var>^3</code>
+                    Since each cube is a cubic <var>UNIT</var>, the volume of the rectangular prism is <code><var>HEIGHT*WIDTH*LENGTH</var> <var>UNIT_TEXT</var>^3</code>
                 </p>
             </div>
         </div>
@@ -332,7 +332,7 @@
             </div>
 
             <p class="question">
-                The volume of this box is <code><var>LENGTH*WIDTH*HEIGHT</var></code> cubic units.
+                The volume of this rectangular prism is <code><var>LENGTH*WIDTH*HEIGHT</var></code> cubic units.
                 What is the number labeled <code>x</code> below?
             </p>
 
@@ -494,7 +494,7 @@
                 </div>
 
                 <p>
-                    Since the slabs have a height of <code>1</code> <var>UNIT</var>, <code>x</code> is the number of these slabs which fill up the whole box.
+                    Since the slabs have a height of <code>1</code> <var>UNIT</var>, <code>x</code> is the number of these slabs which fill up the whole rectangular prism.
                 </p>
                 <p>
                     We can figure out the number of slabs needed by seeing how many times <code><var>LENGTH*WIDTH</var></code>