Correct SDL_ttf CMake module name

Current(incorrect):

CMake Warning at CMakeLists.txt:292 (FIND_PACKAGE):
  By not providing "FindSDL_TTF.cmake" in CMAKE_MODULE_PATH this project has
  asked CMake to find a package configuration file provided by "SDL_TTF", but
  CMake did not find one.

  Could not find a package configuration file provided by "SDL_TTF" with any
  of the following names:

    SDL_TTFConfig.cmake
    sdl_ttf-config.cmake

  Add the installation prefix of "SDL_TTF" to CMAKE_PREFIX_PATH or set
  "SDL_TTF_DIR" to a directory containing one of the above files.  If
  "SDL_TTF" provides a separate development package or SDK, be sure it has
  been installed.

Expected(correct):

-- Found SDL_ttf: /usr/lib/libSDL_ttf.so (found version "2.0.11")