docs(tutorial/4 - Directory and File Organization): change typo/grammar

[TheRealMaxion]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Replace a single word to correct typo/grammar

What is the current behavior? (You can also link to an open issue here)

What is the new behavior (if this is a feature change)?

Does this PR introduce a breaking change?

No.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

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[TheRealMaxion]

I signed it!

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