_content/doc/tutorial/generics.md: fix typo

[mat72coke]

Fixed a minor typo in a comment in the completed code.

There is no issue related to this PR.

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Message from Oleksandr Redko:

Patch Set 1: Code-Review+1

(1 comment)

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Message from Oleksandr Redko:

Patch Set 1:

(1 comment)

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Message from Sean Liao:

Patch Set 1:

(1 comment)

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