Fix typo in macros.md (#1598)

Author: gkevinyen5418

src/macros.md

@@ -37,4 +37,4 @@ So why are macros useful?
 
 3. Variadic interfaces. Sometimes you want to define an interface that takes a
    variable number of arguments. An example is `println!` which could take any
-   number of arguments, depending on the format string!. (More on this later)
+   number of arguments, depending on the format string. (More on this later)