dynamic-array: fix documentation of lttng_dynamic_pointer_array_get_pointer

The documentation of this function says that mutating the array
invalidates the return pointer.  This is not true, since the returned
pointer is the element itself, not a pointer to the array.

Change-Id: I8b8978cddb2d1d6ce0b42ed313e9843ca418c96c
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>