cmake: Fix case sensitive string (PHYSX->Physx)

CMakeLists.txt

@@ -9,7 +9,7 @@ find_package(OpenAL REQUIRED)
 find_package(PythonLibs REQUIRED)
 find_package(EXPAT REQUIRED)
 find_package(ZLIB REQUIRED)
-find_package(PHYSX REQUIRED)  #TODO: Not required if we aren't building the client
+find_package(PhysX REQUIRED)  #TODO: Not required if we aren't building the client
 find_package(Ogg REQUIRED)    #TODO: Not required if we aren't building the client
 find_package(Vorbis REQUIRED) #TODO: Not required if we aren't building the client
 find_package(Speex REQUIRED)  #TODO: Not required if we aren't building the client