C41: Fix path to Understanding image (#4248)

Fixes #4244.

There are exactly two links from Techniques pages to images residing
under Understanding: one in C41, and one in G207.

- The link in G207 correctly omits the version subfolder, which doesn't
exist in the output structure
- The link in C41 did not omit this, causing the link to break

The link in C41 also used uppercase U in Understanding, which would fail
on case-sensitive filesystems. (The inverse does not adversely impact
builds published to w3.org.)


[Preview](https://deploy-preview-4248--wcag2.netlify.app/techniques/css/c41)

Co-authored-by: Mike Gower <mikegower@gmail.com>

Author: kfranqueiro

techniques/css/C41.html

@@ -47,7 +47,7 @@ <h4>CSS</h4>
 				</section>
 
                 <figure id="focus-indicator-inside">
-                    <img src="../../Understanding/22/img/focus-indicator-innerline-strong.png" alt="Three blue buttons with a dark border, the middle button showing a bright yellow outline inside the button." width="300"/>
+                    <img src="../../understanding/img/focus-indicator-innerline-strong.png" alt="Three blue buttons with a dark border, the middle button showing a bright yellow outline inside the button." width="300"/>
                     <figcaption>The default and focused states of the button.</figcaption>
                 </figure>