Rename a 2013/cable3 screenshot file name

As the others files do not specify macOS but are still screenshots of
macOS I have removed the '_on_macosx' from the filename. Thus
2013/cable3/screenshot_win3_on_macosx.png is now screenshot_win3.png.

Makefile updated for this.

2013/cable3/index.html

@@ -736,7 +736,7 @@ the impatient:</p>
 <p><img src="screenshot_simcity.png"
  alt="screenshot image from the Sim City game"
  width=748 height=400></p>
-<p><img src="screenshot_win3_on_macosx.png"
+<p><img src="screenshot_win3.png"
  alt="screenshot image of Windows 3.00a under macOS"
  width=748 height=400></p>
 <h3 id="compiler-warnings">Compiler warnings</h3>
@@ -779,7 +779,7 @@ the source size down. Other compilers are likely to produce similar warnings.</p
 <li><a href="screenshot_lotus123.png">screenshot_lotus123.png</a> - Screenshots of some applications running on macOS</li>
 <li><a href="screenshot_qbasic.png">screenshot_qbasic.png</a> - Screenshots of some applications running on macOS</li>
 <li><a href="screenshot_simcity.png">screenshot_simcity.png</a> - Screenshots of some applications running on macOS</li>
-<li><a href="screenshot_win3_on_macosx.png">screenshot_win3_on_macosx.png</a> - Screenshots of some applications running on macOS</li>
+<li><a href="screenshot_win3.png">screenshot_win3.png</a> - Screenshots of some applications running on macOS</li>
 <li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2013/cable3/sc-ioccc.terminal">sc-ioccc.terminal</a> - config for macOS to correctly display ANSI graphics apps</li>
 <li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2013/cable3/bios.asm">bios.asm</a> - BIOS source in assembly</li>
 </ul>