Update subject

subjects/tetrisoptimizer.en.md

@@ -11,8 +11,8 @@ The program must :
 -   Be written in Go
 -   Compile successfully
 -   Assemble all of the tetrominoes in order to create the smallest square possible
--   Identify each tetromino in the solution, by assigning different letters to different tetrominoes
--   Expect at least 2 tetrominoes in the text file
+-   Identify each tetromino in the solution by printing them with uppercase latin letters (`A` for the first one, `B` for the second, etc)
+-   Expect at least one tetromino in the text file
 -   In case of bad format on the tetrominoes or bad file format it should print `ERROR`
 
 #### Example of a text File