macros.md: Fix htmlized variant of first source code example

Confusingly, the last part of the first source code example would
appear as

def showExpr(expr: Expr[Boolean])(using Quotes): Expr[String] =
  '{ "

         " }

in the htmlized version of the markdown documentation source. Which
reads as follows:

def showExpr(expr: Expr[Boolean])(using Quotes): Expr[String] =
  '{ "<some source code>" } // Better implementation later in this document

This seems to be cause by the usage of angle brackets. Also note that
the code comment does not appear in the htmlized output.

Author: Flowdalic

docs/docs/reference/metaprogramming/macros.md

@@ -40,7 +40,7 @@ def assertImpl(expr: Expr[Boolean])(using Quotes) = '{
 }
 
 def showExpr(expr: Expr[Boolean])(using Quotes): Expr[String] =
-  '{ "<some source code>" } // Better implementation later in this document
+  '{ [actual implementation later in this document] }
 ```
 
 If `e` is an expression, then `'{e}` represents the typed