Eliminating ambiguity of "XML"

[awxiaoxian2020]

This paragraph describes not using XML as a configuration file. So the single word "XML" refers to a configuration file, not a mapper in the context. So when we talk about using XML mappers, we should be explicit.

[coveralls]

Coverage remained the same at 87.227% when pulling f8d4123 on awxiaoxian2020:awxiaoxian2020-patch-2 into 444fed4 on mybatis:master.

[hazendaz]

@awxiaoxian2020 Thank you!

[awxiaoxian2020]

@awxiaoxian2020 Thank you!

Thanks for merging!
I think I can sync it for Chinese doc. I will commit the changes in my PR #2506. They are both about the getting-started page. @hazendaz