load_mon: reduce FDS_LOW_WARNING_THRESHOLD to 3

This bound can be quite tight, because:
- The system still behaves well, even if all FD's are used (as opposed to
  a stack overflow)
- The amount of used FD's is typically only increased one at a time
  (e.g. adding new logged topics, adding a mavlink stream, ...)
- reducing CONFIG_NFILE_DESCRIPTORS to the minimum frees up a considerable
  amount of RAM on systems that need it

src/modules/load_mon/load_mon.cpp

@@ -62,7 +62,7 @@
 extern struct system_load_s system_load;
 
 #define STACK_LOW_WARNING_THRESHOLD 300 ///< if free stack space falls below this, print a warning
-#define FDS_LOW_WARNING_THRESHOLD 10 ///< if free file descriptors fall below this, print a warning
+#define FDS_LOW_WARNING_THRESHOLD 3 ///< if free file descriptors fall below this, print a warning
 
 namespace load_mon
 {