Fix typo in contour transform docs.

Merge pull request vega#784 from vega/v3.0.0-beta.30

v3.0.0-beta.30

Merge pull request vega#775 from vega/dom/startpage

Center images instead of left aligning (better for large screens)

Merge pull request vega#769 from vega/v3.0.0-beta.28

v3.0.0-beta.28

Merge pull request vega#761 from vega/v3.0.0-beta.27

v3.0.0-beta.27

Merge pull request vega#760 from vega/v3.0.0-beta.26

v3.0.0-beta.26

Merge pull request vega#752 from vega/v3.0.0-beta.25

v3.0.0-beta.25

Merge pull request vega#750 from vega/v3.0.0-beta.24

v3.0.0-beta.24

Update README.md.

Merge pull request vega#739 from vega/v3.0.0-beta.22

v3.0.0-beta.22