docs/manual/contribute.txt: fix typo

The directory that containts tests is "support/testing/tests/", not
"supporting/testing/test".

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

docs/manual/contribute.txt

@@ -609,7 +609,7 @@ be the maintainer of that test case.
 
 Within the Buildroot repository, the testing framework is organized at the
 top level in +support/testing/+ by folders of +conf+, +infra+ and +tests+.
-All the test cases live under the +test+ folder and are organized in various
+All the test cases live under the +tests+ folder and are organized in various
 folders representing the category of test.
 
 Lets walk through an example.