Update CMAKE_PREFIX_PATH to CMAKE_INSTALL_PREFIX

[pr0g]

Description of changes

This fixes what I believe is a small mistake/typo in the README.

In the CMake configure command, previously -DCMAKE_PREFIX_PATH was provided which is used to tell CMake where to look for installed libraries (not where to install the library that will be built). For that, you need to use -DCMAKE_INSTALL_PREFIX which I believe it what was intended here.

The flow is usually something like this...

cd aws-sdk-cpp
cmake -B build -DCMAKE_INSTALL_PREFIX=../aws-demo/install -DBUILD_ONLY="s3"
cmake --build build --target install
cd ../aws-demo
cmake -B build -DCMAKE_PREFIX_PATH=install
cmake --build build

There's a chance I've misunderstood and the intention was to find existing libraries, if so please ignore this.

Thanks!

(For reference see the CMake docs CMAKE_PREFIX_PATH and CMAKE_INSTALL_PREFIX.

Check all that applies

• [x] Did a review by yourself.
• [ ] Added proper tests to cover this PR. (If tests are not applicable, explain.)
• [ ] Checked if this PR is a breaking (APIs have been changed) change.
• [ ] Checked if this PR will not introduce cross-platform inconsistent behavior.
• [x] Checked if this PR would require a ReadMe/Wiki update.

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[pr0g]

Thank you for approving. Is there anything else needed before it can be merged? Thanks!