Adjusted filename in git merge output

The previous operations on iss53 were clearly on index.html, so the
merge output should reflect that too for consistency. :)

Author: justinclift

book/03-git-branching/sections/basic-branching-and-merging.asc

@@ -155,7 +155,7 @@ $ git checkout master
 Switched to branch 'master'
 $ git merge iss53
 Merge made by the 'recursive' strategy.
-README |    1 +
+index.html |    1 +
 1 file changed, 1 insertion(+)
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