Update 'practical macros' section to use '=>' instead of '...' syntax…

[strangedreamsNY]

Edited this chapter slightly to ensure as much code as possible is valid throughout or where it is not, ensure that any errors are 'obvious' and not distracting of the chapters intent. Specifically, I updated the example code to use => instead of ... in the macro pattern match. The later value is not actually usable without additional delimiters and results in misleading errors in code that references the macro. This can lead a reader to think they don't correctly understand the parsing structure and clarification doesn't happen until much later in the chapter, rendering all of the previous code examples effectively unusable if they're (likely) following along.