For really quick fixes:
Browse the documentation source code directly on the GitHub website:
Navigate to the file you wish to change and click the edit icon
Use the editor to modify the file
Scroll to the bottom of the page, provide a commit comment and submit.
Create a fork and submit a pull request on your behalf; or
Immediately make the change for those with commit access
This technique is great for fixing small typos - but has the danger of introducing formatting mistakes preventing the documentation from being generated.
To make extensive changes see Workflow.