bug - incorrect sourceLink at the top of md files causes invalid github link in example footers

[nicolethoen]

WindowScroller.md & VirtualizedTable.md both need to have their sourcelinks updated to point to the files in the new react-virtualized-extension repo url.

Then we need to bump the version of react-virtualized-extension released after this update to fix the issue in the extensions-staging.patternfly.org site.