Fix code format and spelling errors in tutorial

[mincong-h]

Hi GWT team,

Here's a trivial fix for code formatting and spelling errors found in GWT Tutorials.

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This change is 

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[mincong-h]

I signed it!

[googlebot]

CLAs look good, thanks!

[jnehlmeier]

Beside the comment, nice work!

[jnehlmeier]

Would you mind changing // <span style="color:black;">**(1)**</span> to // (1) and making sure these comments start on the same column (there are a few more below)?

Currently it renders bad as well: http://www.gwtproject.org/doc/latest/tutorial/JSON.html

[mincong-h]

I didn't change them because I didn't know which formatting to choose. You proposal is a good idea. Just fixed it up.

[tbroyer]

Thank you!

[mincong-h]

My pleasure :)