Fix typo (bad filename) in 2004/arachnid

I had renamed the arachnid.info to arachnid.txt as it's not really an
informative file but I forgot to update the command in the README.md
file. I was tempted to rename it to arachnid.maz as it's a maze file but
I kept it as a text file so that (some?) browsers know what to do with
it (though I'm not sure how strictly necessary that is it doesn't hurt
to have it as .txt either).

2004/arachnid/README.md

@@ -19,7 +19,7 @@ code](#alternate-code) below.
 ## Try:
 
 ```sh
-./arachnid arachnid.info
+./arachnid arachnid.txt
 ```
 
 and then try:

thanks-for-help.md

@@ -2930,6 +2930,11 @@ allows those like himself used to `h`, `j`, `k` and `l` movement keys to not get
 lost. Non rogue players, vi users and Dvorak typists are invited to get lost (or
 use the original version)! :-)
 
+Cody also renamed the `arachnid.info` file to
+[arachnid.txt](/2004/arachnid/arachnid.txt) as it's not really an informative
+file but a maze file. The extension `.maz` was not chosen to help with (some?)
+browsers knowing what to do with it.
+
 
 ## <a name="2004_burley"></a>[2004/burley](/2004/burley/burley.c) ([README.md](/2004/burley/README.md]))