gitmodules: Remove unneeded prefix

Co-authored-by: Morph <39850852+Morph1984@users.noreply.github.com>

.gitmodules

@@ -40,6 +40,6 @@
 [submodule "ffmpeg"]
 	path = externals/ffmpeg
 	url = https://git.ffmpeg.org/ffmpeg.git
-[submodule "externals/SDL"]
+[submodule "SDL"]
 	path = externals/SDL
 	url = https://github.com/libsdl-org/SDL.git