Update the chessboard exercise instructions. (#1674)

Author: bobtfish

exercises/concept/chessboard/.docs/instructions.md

@@ -6,43 +6,43 @@ Each square of the chessboard is identified by a letter-number pair. The vertica
 
 ## 1. Given a Chessboard and a Rank, count how many squares are occupied
 
-Decorate the `Chessboard` type with the `CountInRank(rank int) int` function.
+Implement the `CountInRank(board Chessboard, rank int) int` function.
 It should count occupied squares ranging over a map. Return an integer.
 
 ```go
-CountInRank(1)
+CountInRank(board, 1)
 // => 6
 ```
 
 ## 2. Given a Chessboard and a File, count how many squares are occupied
 
-Decorate the `Chessboard` type with the `CountInFile(file int) int` function.
+Implement the `CountInFile(board Chessboard, file int) int` function.
 It should count occupied squares ranging over the given file. Return an integer.
 
 ```go
-CountInFile(2)
+CountInFile(board, 2)
 // => 5
 ```
 
 ## 3. Count how many squares are present in the given chessboard
 
-Decorate the `Chessboard` type with the `CountAll() int` function.
+Implement the `CountAll(board Chessboard) int` function.
 It should count how many squares are present in the chessboard and returns
 an integer. Since you don't need to check the content of the squares,
 consider using range omitting both `index` and `value`.
 
 ```go
-CountAll()
+CountAll(board)
 // => 64
 ```
 
 ## 4. Count how many squares are occupied in the given chessboard
 
-Decorate the `Chessboard` type with the `CountOccupied() int` function.
+Implement the `CountOccupied(board Chessboard) int` function.
 It should count how many squares are occupied in the chessboard.
 Return an integer.
 
 ```go
-CountOccupied()
+CountOccupied(board)
 // => 15
 ```