fix(docs): change port to targetPort when framework is #custom

[erezrokah]

Fixes the docs to match https://github.com/netlify/cli/blame/master/docs/netlify-dev.md#L151,

cli/src/utils/detect-server.js

Line 78 in 8efe94e

throw new Error('"command" and "targetPort" properties are required when "framework" is set to "#custom"')

,

cli/src/utils/detect-server.js

Line 82 in 8efe94e

'"framework" option must be set to "#custom" when specifying both "command" and "targetPort" options',

and #843 (comment)