fix(cli): Display only one, correct default for --preset flag

[pioluk]

Previously --help would print both "angular" and "conventionalcommits" as defaults for --preset, which was confusing.

$ standard-version --help
Before:

  --preset             Commit message guideline preset (default: angular)
                                                        [string] [default:"conventionalcommits"]

After:

  --preset             Commit message guideline preset  [string] [default: "conventionalcommits"]

[coveralls]

Coverage remained the same at 100.0% when pulling 1eef66a on pioluk:fix/display-correct-default-for-preset into 3aaf72e on conventional-changelog:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 1eef66a on pioluk:fix/display-correct-default-for-preset into 3aaf72e on conventional-changelog:master.

[bcoe]

@pioluk thanks for the contribution 👍