Fix IMGUI_SFML_CONFIG_INSTALL_DIR
Fix IMGUI_SFML_CONFIG_INSTALL_DIR

* Fix comment (IMGUI_SFML_INSTALL_CONFIG_DIR -> IMGUI_SFML_CONFIG_INSTALL_DIR)
* Only add IMGUI_SFML_CONFIG_INSTALL_DIR to include directories if IMGUI_SFML_CONFIG_INSTALL_DIR is set
eliasdaler/imgui-sfml,EliasD/imgui-sfml,eliasdaler/imgui-sfml