changing typo 'lamba' to 'lambda' (line 2596)

"links = [dict(header='Virtual Field 1',body=lamba row:row.vfield),...] )" changed to "links = [dict(header='Virtual Field 1',body=lambda row:row.vfield),...] )"
Merge pull request #242 from spokes8249/patch-1

changing typo 'lamba' to 'lambda' (line 2596 in chapter 7)