Update README.md (opnsense#580)

Since make clean does "rm -rf build/*", the sphinx-autobuild should happen in "build/html".

Author: Monviech

README.md

@@ -71,5 +71,5 @@ pip[3] install sphinx-autobuild
 ```
 
 ```
-sphinx-autobuild source html
+sphinx-autobuild source build/html
 ```