Provide correct directory for HTML documentation output

Updated output directory in tutorial for manual sphinx-build operations
on godot-docs. It now matches the output directory used by the make
command for godot-docs.

contributing/documentation/building_the_manual.rst

@@ -103,10 +103,10 @@ program manually:
 
 .. code:: sh
 
-   sphinx-build -b html ./ _build
+   sphinx-build -b html ./ _build/html
 
 You can also specify a list of files to build, which can greatly speed up compilation:
 
 .. code:: sh
 
-  sphinx-build -b html ./ _build classes/class_node.rst classes/class_resource.rst
+  sphinx-build -b html ./ _build/html classes/class_node.rst classes/class_resource.rst