Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/aws/exercises/placement_groups/exercise.md

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+## AWS EC2 - Placement Groups          
+
+### Objectives                      
+
+A. Create a placement group. It should be one with a low latency network. Make sure to launch an instance as part of this placement group.
+B. Create another placement group. This time high availability is a priority

topics/aws/exercises/placement_groups/solution.md

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+## AWS EC2 - Placement Groups          
+
+### Objectives                      
+
+A. Create a placement group. It should be one with a low latency network. Make sure to launch an instance as part of this placement group.
+B. Create another placement group. This time high availability is a priority
+
+### Solution                        
+
+A.
+1. Go to EC2 service
+2. Click on "Placement Groups" under "Network & Security"
+3. Click on "Create placement group"
+4. Give it a name and choose the "Cluster" placement strategy because the requirement is low latency network
+5. Click on "Create group"
+6. Go to "Instances" and click on "Launch an instance". Choose any properties you would like, just make sure to check "Add instance to placement group" and choose the placement group you've created
+
+B.
+1. Go to EC2 service
+2. Click on "Placement Groups" under "Network & Security"
+3. Click on "Create placement group"
+4. Give it a name and choose the "Spread" placement strategy because the requirement is high availability as top priority
+5. Click on "Create group"