Placeholder mobile application example is live on docs

[ztaylor54]

In the how-to's, there is a placeholder for "how to build a mobile application" which just repeats the market time example. This might be confusing for some users.

Link: https://docs.alpaca.markets/api-documentation/how-to/apps/#use-the-api-to-build-a-mobile-application

Affected files: https://github.com/alpacahq/user-docs/blob/master/content/api-documentation/how-to/apps/_index.md