You shouldn't rename the file because, there's no mispelling: the name used by most BGLs in the scenery IS fx_FSDT_PHNLGT.fx so, if you rename it, those BGL will not be find the effect they are looking for.
However, there are a couple of BGL that are referencing the version with two 'L', which is not correct but, to fix it in the right way, you should make a copy instead of the fx_FSDT_PHNLGT.fx to fx_FSDT_PHNLLGT.fx
Until we'll fix the BGL, this will fix the problem and will have any side effect, other than taking an additional 1275 bytes of hard drive space...