Fix intro-to-module-scripts.md having a mixup

[literallytwo]

Changes

to be specific, it says to create a ModuleScript in ServerStorage, but it is previously stated that they are commonly placed in ServerScriptService, so this fixes it

No links are needed, as the file and image right below the line I changed both state that it should be in ServerScriptService

Checks

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[github-actions]

Hi @literallytwo, thanks so much for helping improve the Roblox creator documentation! Our technical writing team will review your pull request soon. In the meantime, please ensure you've read through the README.md, contribution guidelines, and style recommendations.