Makefile: fix the location of docs API ReST files

The API ReST files that are generated dinamycally are actually put in
the source directory, and not in the build directory. This fixes a
mistake by yours truly.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

Makefile

@@ -58,7 +58,7 @@ clean:
 	rm -f man/avocado.1
 	rm -f man/avocado-rest-client.1
 	rm -rf docs/build
-	rm -f docs/build/api/*.rst
+	rm -f docs/source/api/*.rst
 	test -L avocado/virt && rm -f avocado/virt || true
 	test -L avocado/plugins/virt.py && rm -f avocado/plugins/virt.py || true
 	test -L avocado/plugins/virt_bootstrap.py && rm -f avocado/plugins/virt_bootstrap.py || true