qapi/block.json: Add newline after "Example:" for block-latency-histogram-set

The block-latency-histogram-set command is the only one which uses
the Example/Examples section with the first line of the documentation
immediately following the ':'. Bring it into line with the rest.

This will allow us to avoid special-casing the indentation handling
for "Examples" sections; instead for Examples as for any other section
header these two indentations will be equivalent:

Examples:
Line one
Line two

Examples: Line one
          Line two

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200925162316.21205-3-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>