doc/gitcore-tutorial: fix prose to match example command

In 328c6cb853 (doc: promote "git switch", 2019-03-29), an example
was changed to use "git switch" rather than "git checkout" but an
instance of "git checkout" in the explanatory text preceding the
example was overlooked. Fix this oversight.

Signed-off-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitcore-tutorial.txt

@@ -751,7 +751,7 @@ to it.
 ================================================
 If you make the decision to start your new branch at some
 other point in the history than the current `HEAD`, you can do so by
-just telling 'git checkout' what the base of the checkout would be.
+just telling 'git switch' what the base of the checkout would be.
 In other words, if you have an earlier tag or branch, you'd just do
 
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