small grammar fix: it have -> it has

Nils Reuße · Nils Reuße · commit 9139ee0a9e58 · 2015-01-22T23:02:40.000+01:00
diff --git a/book/07-git-tools/sections/revision-selection.asc b/book/07-git-tools/sections/revision-selection.asc
@@ -85,7 +85,7 @@ A higher probability exists that every member of your programming team will be a
 [[_branch_references]]
 ==== Branch References
 
-The most straightforward way to specify a commit requires that it have a branch reference pointed at it.
+The most straightforward way to specify a commit requires that it has a branch reference pointed at it.
 Then, you can use a branch name in any Git command that expects a commit object or SHA-1 value.
 For instance, if you want to show the last commit object on a branch, the following commands are equivalent, assuming that the `topic1` branch points to `ca82a6d`:
 
