[FIX] typo in Renderer.extend documentation
[FIX] typo in Renderer.extend documentation

Renderers extended from sap.ui.core.Renderer must implement render function

Closes https://github.com/SAP/openui5/pull/1042
Change-Id: I6f63f2e1df47b4c48c73f4143bad0c63cbf36b7f

cschuff/openui5,olirogers/openui5,SAP/openui5,SQCLabs/openui5,SAP/openui5,SQCLabs/openui5,SQCLabs/openui5,olirogers/openui5,olirogers/openui5,SAP/openui5,SAP/openui5,nzamani/openui5,nzamani/openui5,cschuff/openui5,cschuff/openui5,nzamani/openui5