Minor fixes in docs.

[jarimatti]

Minor fixes in doc Markdown.

• Prometheus.Metric doc indent fixed to format as code
• Format Prometheus.Metrics.Histogram bucket generate specs as code

[jarimatti]

Hmm... check.sh "works on my machine". I'll see if I can reproduce the CI build failure.

[jarimatti]

The CI build fails with Elixir 1.3 and 1.4 on Credo, in file gauge_test.exs. It seems to bark on negative number in line 250:

assert -8.5 == Gauge.value(spec)

So probably nothing to do with the changes in this pull request.

[deadtrickster]

Thanks!