updated simpsonsIcons.css - cap S on Simpsons.gif

In our automated build process for Dojo 1.81, when using the cssOptimize flag we ran into issues related to this file due to the capitalization in the css file not matching the actual file. 

We were getting this error from the Dojo build script:
<pre>
starting executing global optimizations...
error(357) While optimizing a CSS file, it was impossible to compute the destination location of a relative URL. 
CSS file: C:/dev/workspace/myproject/WebContent/thirdparty/cbtree/icons/cbtreeIcons.css; 
import: C:/dev/workspace/myproject/WebContent/thirdparty/cbtree/icons/simpsonsIcons.css; 
relative URL: url('images/simpsons.gif')
</pre>

Making this small change fixes the issue.