docs(halfSlice): fix make title in kebab-case and fix indentation.

subjects/halfslice/README.md

@@ -1,4 +1,4 @@
-## half-Slice
+## half-slice
 
 ### Instructions
 
@@ -25,7 +25,7 @@ import (
 
 func main() {
 	fmt.Println(HalfSlice([]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}))
-    fmt.Println(HalfSlice([]int{1, 2, 3}))
+	fmt.Println(HalfSlice([]int{1, 2, 3}))
 }
 ```
 
@@ -35,4 +35,4 @@ And its output :
 $ go run .
 [1,2,3,4,5]
 [1,2]
-```
+```