add nested array indexing example

[kevpl]

This PR adds an example to the docs to help people who've hit the snag
discussed in #1168.

Having an example allows people who hit this snag to relatively quickly
see what they've done wrong, rather than having to probe further into a
cryptic error message:

jq: error: syntax error, unexpected $end, expecting FORMAT or QQSTRING_START (Unix shell quoting issues?)

Having it just as an example allows people to be informed of the behavior
without the burden of needing to explain it with additional text.

[coveralls]

Coverage increased (+0.07%) to 84.641% when pulling 037f8ba on kevpl:array_nested_example into 6df393c on stedolan:master.

[nicowilliams]

I mean, the manual is littered with examples like this, but I guess the point is that just the section on array indexing can confuse people, so I'll take it.

[nicowilliams]

This needs to be output: ['"b"'].

[itchyny]

The example output needs to be corrected as commented before.