Unexpected colon in cradle example.

[philderbeast]

From the readme, trying the cradle: {stack:} example for hie.yaml I get an error with hie-core and yamllint.

https://github.com/mpickering/hie-bios/blame/master/README.md#L38

> hie-core
Starting hie-core
...
[1/6] Finding hie-bios cradle
hie-core: InvalidYaml (Just (YamlParseException {yamlProblem = "found unexpected ':'"
, yamlContext = "while scanning a plain scalar"
, yamlProblemMark = YamlMark {yamlIndex = 14, yamlLine = 0, yamlColumn = 14}}))

Trying yamllint

> cradle: {stack:}
found unexpected ':' while scanning a plain scalar at line 1 column 10

[mpickering]

Thanks, fixed.