docs/manual/contribute.txt: fix typo

Signed-off-by: Raphaël Mélotte <raphael.melotte@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 226d5b3b95c1c54449c31358c888640281d45a33)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/contribute.txt

@@ -610,7 +610,7 @@ 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
-folders representing the catagory of test.
+folders representing the category of test.
 
 Lets walk through an example.