Remove <code> elements from mdx stories

For some reason these <code> elements cause an error 'children.match is not
a function' and prevent the page from rendering. Might be related to
storybookjs/storybook#15359. I don't have time to
find a fix for this, so I'm just switching to <span>s for now.

src/Utils/Shadow/Shadows.stories.mdx

@@ -45,10 +45,10 @@ const MyCard = styled.div`
         <tr>
           <td rowSpan={2}>Level {index + 1}</td>
           <td>
-            <code>up{index + 1}</code>
+            <span>up{index + 1}</span>
           </td>
           <td>
-            <code>{level[0]}</code>
+            <span>{level[0]}</span>
           </td>
           <td rowSpan={2}>
             <PreviewCell>
@@ -60,10 +60,10 @@ const MyCard = styled.div`
         </tr>
         <tr>
           <td>
-            <code>down{index + 1}</code>
+            <span>down{index + 1}</span>
           </td>
           <td>
-            <code>{level[1]}</code>
+            <span>{level[1]}</span>
           </td>
         </tr>
       </Fragment>

src/stories/Introduction.stories.mdx

@@ -6,7 +6,7 @@ import { AriesStatus } from './story-components/AriesStatus';
 
 # Glints Aries
 
-<code>v{version}</code>
+<span>v{version}</span>
 
 Glints Aries is Glints' in-house React component library. Most components are specific to Glints but you're free to use them in your own projects or to fork this project to build your own component library. We also host a number of non-component code units such as colors and breakpoints.