Update MP SELECT Descendants.md

Author: pchemguy

MP SELECT Descendants.md

@@ -113,9 +113,11 @@ SELECT * FROM records ORDER BY path;
 
 in which %SELECTOR% should be replaced according to the following table
 
+<pre>
 | node set    | %SELECTOR%                          |
 |-------------|-------------------------------------|
 | root nodes  | `cats.path    IN  (tops.path)`      |
 | descendants | `cats.prefix like tops.path || '%'` |
 | subtrees    | `cats.path   like tops.path || '%'` |
 | children    | `cats.prefix  =   tops.path`        |
+</pre>