Remove trailing slash in osa submodule

Remove trailing slash in osa submodule

Gerrit is not properly updating doc repo when we merge patches
in osa repo.

My current theory is that gerrit cannot properly match
a submodule to gerrit project as in .gitmodules project name is
written with trailing slash. As it's the only submodule that has this
trailing slash let's give it a try and remove that slash.

Issue-ID: SECCOM-246
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Change-Id: I578432a6518c94ad8911202542a9a037acc60db5