fix introduction.md in 'assembly-line' exercise

[takaho9]

Please see the forum
http://forum.exercism.org/t/misinfo-in-ruby-concept-exercise/12633/2

[github-actions]

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[kotp]

Approving, since, while the statement is not false, the resulting change does indicate how it should be done, without giving an opinion about it definitively. In other words, leaving out the fact that you can also avoids encouraging doing so.

I like it.