typo fix ch06-01-defining-an-enum.md

src/ch06-01-defining-an-enum.md

@@ -105,7 +105,7 @@ a struct. Enums handle this case with ease:
 
 We’ve shown several different ways to define data structures to store version
 four and version six IP addresses. However, as it turns out, wanting to store
-IP addresses and encode which kind they are is so common that [the standard
+IP addresses and encode which kind they are so common that [the standard
 library has a definition we can use!][IpAddr]<!-- ignore --> Let’s look at how
 the standard library defines `IpAddr`: it has the exact enum and variants that
 we’ve defined and used, but it embeds the address data inside the variants in