Documentation: Recommend setting PREFIX_PATH instead of INSTALL_PREFIX

This was straight up the wrong CMake variable to set. We care about
where the project will find Lagom, not where it wants to install its own
binaries to.

Documentation/CLionConfiguration.md

@@ -14,7 +14,7 @@ After opening the `serenity` repository in CLion as a new project, the "`Open Pr
 ```
 -GNinja
 -DCMAKE_TOOLCHAIN_FILE=$CMakeProjectDir$/Build/i686/CMakeToolchain.txt
--DCMAKE_INSTALL_PREFIX=$CMakeProjectDir$/Build/lagom-install
+-DCMAKE_PREFIX_PATH=$CMakeProjectDir$/Build/lagom-install
 -DSERENITY_ARCH=i686
 ```