Align package name to FindFlatBuffers.cmake (#5899)

Align package name to FindFlatBuffers.cmake (#5899)

This change is for fix warning:
```
CMake Warning (dev) at /home/aoleksy/share/cmake-3.17/Modules/FindPackageHandleStandardArgs.cmake:272 (message):
  The package name passed to `find_package_handle_standard_args`
  (flatbuffers) does not match the name of the calling package (FlatBuffers).
  This can lead to problems in calling code that expects `find_package`
  result variables (e.g., `_FOUND`) to follow a certain pattern.
Call Stack (most recent call first):
  FindFlatBuffers.cmake:33 (find_package_handle_standard_args)
  CMakeLists.txt:6 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.
```