Rollup merge of #29193 - kini:filename-conventions, r=alexcrichton

When reading this paragraph, the beginning Rust programmer is starting
to write a Hello World program.  We have just told her to name the file
`main.rs`, and immediately afterward, a `hello_world.rs` is mentioned.
I changed this to an unrelated filename (incidentally one that appears
in this repository) to make it clear that this is just an example.
Also, wording it as a declarative sentence rather than an imperative one
further separates it from the Hello World instructions in this section.

r? @steveklabnik 

(Let me know if I'm sending too many PRs -- I can batch up TRPL edits, say, per chapter, if that works better. Or I can just refrain from editing TRPL as I read through it, if these are not sufficiently useful.)

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