Fix link to Design Principles in PTransform Style Guide

The href to Design Principles was broken in the PTransform Style
Guide in the documentation. The href was changed to point to the
Contribution Guide.

website/src/contribute/ptransform-style-guide.md

@@ -30,7 +30,7 @@ _A style guide for writers of new reusable PTransforms._
 ### Consistency
 Be consistent with prior art:
 
-* Adhere to [Beam design principles]({{ site.baseurl }}/contribute/design-principles/).
+* Adhere to [Beam design principles]({{ site.baseurl }}/contribute/).
 * If there is already a similar transform in some SDK, make the API of your transform similar, so that users' experience with one of them will transfer to the other. This applies to transforms in the same-language SDK and different-language SDKs.
 *Exception:* pre-existing transforms that clearly violate the current style guide for the sole reason that they were developed before this guide was ratified. In this case, the style guide takes priority over consistency with the existing transform.
 * When there is no existing similar transform, stay within what is idiomatic within your language of choice (e.g. Java or Python).