docs: fix a typo

Author: wtlin1228

src/primitives/array.md

@@ -6,7 +6,7 @@ at compile time, is part of their type signature `[T; length]`.
 
 Slices are similar to arrays, but their length is not known at compile time.
 Instead, a slice is a two-word object; the first word is a pointer to the data,
-the second word the length of the slice. The word size is the same as usize,
+the second word is the length of the slice. The word size is the same as usize,
 determined by the processor architecture, e.g. 64 bits on an x86-64. Slices can
 be used to borrow a section of an array and have the type signature `&[T]`.