docs: fix end-to-end example

[guybedford]

This makes the existing example use ComponentizeJS to create the demo component so that it is a fully runnable end to end example.

Resolves #278.

[bakkot]

Nice! Went through it. Ran into the following problems:

• Following the "install" instructions in the main readme will not put jco on your path, so the commands which say to run jco will not work. Users either need to install -g, or prefix all jco commands with npx, as in npx jco componentize cowsay.js --wit cowsay.wit -o cowsay.wasm.
• The first time you run componentize it will fail and tell you that you need to run npm install @bytecodealliance/componentize-js.
• Running the test file will fail because it's trying to import from a file (./cowsay/cowsay.js) which ends in .js, which npm treats as cjs by default. Users either need to be told to create a package.json with "type": "module" (as in the ComponentizeJS example), or the code could generate one, or (my preference) the code could generate .mjs files instead of .js.

(Also, typo "lets create" -> "let's create")

[guybedford]

Thanks for taking a look, added some install instructions and type: module instructions.

[bakkot]

Following this as written worked for me and produced the claimed output. Much nicer!