Rename *.text *.md

As luck would have it the only entries where this applied were my own.
The links/references to these files have been changed in the files.

An unfortunate problem is that in 2020/ferguson1 there are several
*.markdown files. I will have to go through these and figure out which
ones should be changed to .md and which ones can be removed. This can
happen later but in the meantime the references are to .md. I believe
the only difference is that the .md has the footer added by the judges.
This was a workaround with the fact that I wasn't sure (and still am not
entirely sure) the generation of the html files will work. It might be
that the .markdown files have the footer and the .md files do not but
this can be dealt with later anyway.

2018/ferguson/README.md

@@ -1123,7 +1123,7 @@ like to thank my mother for being the best mum anyone could ask for, who's
 always been here for me and whom I also inherit the keen analytical mind from. I
 love you all!
 
-I want to also thank Dave Burton for acknowledging me in his hint.markdown file
+I want to also thank Dave Burton for acknowledging me in his README.md file
 for as he put it 'being relentless with pursuit of bugs'. To think that it was
 circumstantial to my entry too makes it all the more unlikely but that doesn't
 take away the significance of it; for I am a very symbolic person and being