Escaped the book's example {{#include ...}} tag

Author: Michael-F-Bryan

book-example/src/for_developers/preprocessors.md

@@ -4,7 +4,7 @@ A *preprocessor* is simply a bit of code which gets run immediately after the
 book is loaded and before it gets rendered, allowing you to update and mutate
 the book. Possible use cases are:
 
-- Creating custom helpers like `{{#include /path/to/file.md}}`
+- Creating custom helpers like `\{{#include /path/to/file.md}}`
 - Updating links so `[some chapter](some_chapter.md)` is automatically changed 
   to `[some chapter](some_chapter.html)` for the HTML renderer
 - Substituting in latex-style expressions (`$$ \frac{1}{3} $$`) with their