Fix dynamodb example README

Author: Jake Stoeffler

examples/dynamodb/README.md

@@ -13,7 +13,7 @@ $ git clone https://github.com/aws/aws-sdk-cpp.git
 $ cd aws-sdk-cpp
 $ mkdir build
 $ cd build
-$ cmake .. -DBUILD_ONLY="s3" \
+$ cmake .. -DBUILD_ONLY="dynamodb" \
   -DCMAKE_BUILD_TYPE=Release \
   -DBUILD_SHARED_LIBS=OFF \
   -DENABLE_UNITY_BUILD=ON \