docs(data-structures): correct grammar

subjects/mobile-dev/data-structures/README.md

@@ -1,43 +1,45 @@
-# Data Structures
+## Data Structures
 
 ### Instructions
 
-In this exercise you will work with the following data structures: List, Set, and Map.
+Let's take a look at `List`, `Set`, and `Map`.
 
 Declare and initialize following variables:
 
-- `listNum` of type `List<int>` containing a list of integers (list length >= 5);
-- `listObj` of type `List<Object>` containing a list of items of different types (list length >= 4);
-- `listStr` of type `List<String>` containing a list of strings (list length >= 3);
-- `listList` of type `List<List<Object>>` containing a list of lists containing `listNum`, `listObj`, `listStr`;
-- `setStr` of type `Set<String>` containing at least 3 items;
-- `mapStr` of type `Map<String, int>` containing at least 3 pairs;
-
-### Usage
+- `listNum` of type `List<int>`, containing a list of integers. There must be at least 5 elements in the list.
+- `listObj` of type `List<Object>`, containing a list of objects. There must be at least 4 elements in the list.
+- `listStr` of type `List<String>`, containing a list of strings. There should be at least 3 elements in the list.
+- `listList` of type `List<List<Object>>`, containing a list of lists containing `listNum`, `listObj`, `listStr`.
+- `setStr` of type `Set<String>`, containing at least 3 strings.
+- `mapStr` of type `Map<String, int>` containing at least 3 pairs.
 
 ### List
 
-List is an array of elements. In dart lists are initialized as following:
+`List` is an array of elements:
 
 ```dart
-var listNum1 = [1, 2, 3];
-// or ...
-List<int> listNum2 = [1, 2, 3];
+var listNum = [1, 2, 3];
+```
+
+```dart
+List<int> listNum = [1, 2, 3];
 ```
 
 ### Set
 
-Set is an unordered collection of unique items. Sets are created as follows:
+`Set` is an unordered collection of unique items:
+
+```dart
+var set = {'Germany', 'Kazakhstan', 'France', 'England'};
+```
 
 ```dart
-var set1 = {'Germany', 'Kazakhstan', 'France', 'England'};
-// or ...
-Set<String> set2 = {'Germany', 'Kazakhstan', 'France', 'England'};
+Set<String> set = {'Germany', 'Kazakhstan', 'France', 'England'};
 ```
 
 ### Map
 
-Map is a key-value data structure. Maps are created as:
+`Map` is a key-value data structure:
 
 ```dart
 var mapRadius = {
@@ -45,7 +47,9 @@ var mapRadius = {
   'Jupiter': 71492,
   'Moon': 1738.1,
 };
-// or ...
+```
+
+```dart
 Map<String, double> mapRadius = {
   'Earth': 6378.1,
   'Jupiter': 71492,
@@ -53,4 +57,4 @@ Map<String, double> mapRadius = {
 };
 ```
 
-- Note: main is not needed!
+> No main is needed.