Change why is to why are

16-slices/questions/4-backing-array.md

@@ -29,7 +29,7 @@ s[2:] // <-- slicing
 > **3:** Yes! Slicing returns a new slice that references to some segment of the same backing array.
 
 
-## Why is slicing and indexing a slice efficient?
+## Why are slicing and indexing a slice efficient?
 1. Slices are fast
 2. Backing arrays are contiguous in memory *CORRECT*
 3. Go uses clever algorithms