[java/en] Fix minor typo

[ds2643]

This commit fixes a very minor typo in the java (English) tutorial.

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[ds2643]

The previous section introduces var, which allows for type inference. To my knowledge, vat isn't valid Java:

jshell> vat textBlock = "invalid";
|  Error:
|  cannot find symbol
|    symbol:   class vat
|  vat textBlock = "invalid";
|  ^-^

Code snippet above is executed in jshell running on openjdk/11.