moved footnote to after </tbody> in update_html

[nolankm]

Small tweak to get the footnote to display at the bottom of the table when pasted to a spreadsheet application. Previously the footnote displayed before table body. Screenshot shows results before tweak and after tweak using Microsoft Excel:

table1(~mpg+cyl+disp, 
       data=mtcars,
       footnote='This is a test footnote')

Thank you for the package, I use it all the time, it's fantastic!