Add files from 2020/endoh2 password-protected zip

The password was no longer known as the link that had a list of
passwords as provided by Yusuke in 2020 was no longer there, being
instead from 2021. I didn't have to use the Internet Wayback Machine as
I had already extracted them in the 2020 preview back then. The
spoiler/spoiler.md has been format fixed.

The spoiler.zip file has been deleted. The README.md file has been fixed
to refer to the directory, not the zip file, and a missing '`' was added
to the file that prevented proper rendering of the html.

2020/endoh2/README.md

@@ -26,15 +26,16 @@ make
 ./prog yoda.txt
 
 ./prog bear.txt
-``
+```
 
 ## Judges' remarks:
 
-Clever use of the C99 complex data type. The spoiler.zip file contains clues
-as to how some winning authors create / edit programs into ASCII shapes.
+Clever use of the C99 complex data type. The [spoiler](spoiler/) directory
+contains clues as to how some winning authors create / edit programs into ASCII
+shapes.
 
-With the source from the spoiler.zip file could you make the code render for a
-larger terminal window?  How about adding more glyphs?
+With the source from the [spoiler](spoiler/) directory could you make the code
+render for a larger terminal window?  How about adding more glyphs?
 
 ## Author's remarks: