ref-manual: release-process: grammar fix

(From yocto-docs rev: 9c15a33c35ccebf088befce411108cabf7ba3159)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/ref-release-process.rst

@@ -138,7 +138,7 @@ consists of the following pieces:
    piece of software. The test allows the packages to be be run within a
    target image.
 
--  ``oe-selftest``: Tests combination BitBake invocations. These tests
+-  ``oe-selftest``: Tests combinations of BitBake invocations. These tests
    operate outside the OpenEmbedded build system itself. The
    ``oe-selftest`` can run all tests by default or can run selected
    tests or test suites.