Make Immutable.js comparison example more idiomatic and readable

[acusti]

the change makes the Immutable.js example far more readable and illustrates a userland syntax that is comparable to the syntax in this proposal. also, the simplified version actually works (a property path can traverse Maps and Lists), as illustrated here: https://codesandbox.io/s/immutable-js-deep-path-properties-example-xqc9y

also:

• fixed typos (“succient” → “succinct”, “propertiess” → “properties”)
• added a trailing comma to the state1 record in the first example for consistency
• revise the double-negative in “much more unclear” → “much less clear”
• use backticks to render TypeError as monospace