fix(ox_lib/marker): Make example standalone

[Moozdzn]

The current marker example gets the point coords from a non existent SharedConfig and calls GetEntityCoords on what I assume is a vector value.

Changes made

Add a new center local variable that holds a vector3 value (police parking lot coords) Add a new uiText local variable that holds the text to be displayed.

Make use of the second argument returned by isTextUIOpen (the text currently being displayed) to compare it to uiText. This prevents closing the text ui if it has been overriden.