style: no capitalized error messages

pkg/tokenizer/tokenizer.go

@@ -14,7 +14,7 @@ func getToken(i *iterator.Iterator[rune]) (*Token, error) {
 
 	letter, err := i.Next()
 	if err != nil {
-		return nil, fmt.Errorf("Cannot produce next token: %w", err)
+		return nil, fmt.Errorf("cannot produce next token: %w", err)
 	}
 
 	// If it is an operand.