Update the instructions for previewing.

Author: amartini51

CONTRIBUTING.md

@@ -83,7 +83,7 @@ and hard wrapping text automatically.
 
 Use the following steps when creating a new pull request:
 
-1. Test that your changes build locally by running `make preview`.
+1. Test that your changes build locally by running `docc preview TSPL.docc`.
 2. Create a pull request in this repository.
 3. Write a brief message in the pull request to introduce your work in context.
 

README.md

@@ -44,7 +44,7 @@ For more information, see [Contributing to The Swift Programming Language][contr
 
 ## Building
 
-Run `swift package --disable-sandbox preview-documentation --target TSPL`
+Run `docc preview TSPL.docc`
 in this repository's root directory.
 
 After running DocC, open the link that `docc` outputs