Fix subline position term in velocity formula

The subline potion was previously added with incorrect sign (should be
negated instead). This might be partially responsible for animation
speed quirks described in
#22 (comment)

autoload/smoothie.vim

@@ -183,7 +183,7 @@ endfunction
 " TODO: current algorithm is rather crude, would be good to research better
 " alternatives.
 function s:compute_velocity()
-  return g:smoothie_base_speed * (s:target_displacement + s:subline_position)
+  return g:smoothie_base_speed * (s:target_displacement - s:subline_position)
 endfunction
 
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