Fix minor typos and clarify Parsing and evaluation... section

[EjPlatzer]

I found two minor typos in the beginning of the book.

Also, I wanted to try to clarify the Parsing and evaluation... section of Thinking in Nu because I think this is by far the most confusing section for people coming to Nu. I know there have been discussions of how to clarify it on Discord, but I figured I'd submit some minor changes as a first effort.

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Personally, I think that the given example is more confusing than is necessary. This is especially true because if you run each line as a separate command in the shell, it will work perfectly - and this is how most people will think to try the script.

Instead, I would highlight the second example and use that to explain the overall concept. It's much more clear (to me, at least) that sourcing a dynamic path doesn't work because the source command tries to parse the contents of the path before the dynamic expression has been evaluated. We could, for example, give this self-contained example:

> source $nu.config-path

This has the added benefit that it would fail even if run from the shell, unlike the current primary example. Also, I think this is a familiar command to people coming from other shells, because sourcing the [.bashrc/.zshrc/...] is the normal way to reload the configuration values. I think that isn't best practice in Nushell, but this section is just about explaining the concepts, not instructing in the proper use of Nu.

[tikue]

This is especially true because if you run each line as a separate command in the shell, it will work perfectly - and this is how most people will think to try the script.

Thank you! I was banging my head against this for a few minutes trying to understand what I was missing.

[fdncred]

Thanks!