str type: make sentence more readable

Author: tshepang

src/types/textual.md

@@ -8,7 +8,7 @@ or 0xE000 to 0x10FFFF range. It is immediate [Undefined Behavior] to create a
 `char` that falls outside this range. A `[char]` is effectively a UCS-4 / UTF-32
 string of length 1.
 
-A value of type `str` is represented the same way as `[u8]`, it is a slice of
+A value of type `str` is represented the same way as `[u8]`, a slice of
 8-bit unsigned bytes. However, the Rust standard library makes extra assumptions
 about `str`: methods working on `str` assume and ensure that the data in there
 is valid UTF-8. Calling a `str` method with a non-UTF-8 buffer can cause