Fix observer error

Change on weak for strong and this will solve this kind of error:

An instance 0x136e5c510 of class UIScrollView was deallocated while key
value observers were still registered with it. Observation info was
leaked, and may even become mistakenly attached to some other object.
Set a breakpoint on NSKVODeallocateBreak to stop here in the debugger.
Here's the current observation info:
<NSKeyValueObservationInfo 0x170441b30> (
<NSKeyValueObservance 0x1702c6190: Observer: 0x136e1c650, Key path:
contentOffset, Options: <New: YES, Old: NO, Prior: NO> Context: 0x0,
Property: 0x170441bc0>