Improve markdown guidelines wording

Amongst the changes here is that we do not need to have the hex or html
code (numbers) for open and close parentheses (in links) as there are
names. '(' is & followed by lpar; and the ')' is the same but 'rpar;'
instead. This makes it easier to remember and type and it generates the
same html as well (they are all valid). Now the markdown files are
consistent in this (in some cases hex was used and in others the html
code).

Although many markdown files were updated no html files needed to be
updated as it generates the same html. Even so I ran the appropriate
rules to do so just to verify this.

2001/dgbeards/README.md

@@ -160,7 +160,7 @@ program would probably be rated somewhere around 1700-1900.  It is better
 than most players, but will get destroyed by experienced human opponents.
 
 This program searches to a fixed depth of 3
-[ply](https://en.wikipedia.org/wiki/Ply_(game_theory)).  Because it does not
+[ply](https://en.wikipedia.org/wiki/Ply_(game_theory)).  Because it does not
 search very deep, it performs very poorly in quiet strategic positions.
 In such positions, it will often just shuffle pieces around aimlessly
 until a tactical exchange presents itself.