Minor - Update Fig 2.6 to use .bck

[dhunter]

Before and after Figure 2.6, you're talking about filtering out *.bck files, but Fig 2.6, which shows how filtering actually works, is showing files with a trailing tilde instead. Would think it best, for continuity with the surrounding text, to adjust Fig 2.6 to convert "first.txt~" and "second.txt~" to "first.bck" and "second.bck", so that it's crystal clear why a *.bck filter would remove those two files.