doc: fix a typo in adding-syscalls.rst

There was a typo in adding-syscalls.rst that could mislead developers
to add a C filename in a makefile instead of an object filename.
This error, while not keeping developers from contributing could slow
the development process down by introducing build errors.
Signed-off-by: Guillaume Dore <corwin@poussif.eu>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/adding-syscalls.rst

@@ -232,7 +232,7 @@ normally be optional, so add a ``CONFIG`` option (typically to
    by the option.
  - Make the option depend on EXPERT if it should be hidden from normal users.
  - Make any new source files implementing the function dependent on the CONFIG
-   option in the Makefile (e.g. ``obj-$(CONFIG_XYZZY_SYSCALL) += xyzzy.c``).
+   option in the Makefile (e.g. ``obj-$(CONFIG_XYZZY_SYSCALL) += xyzzy.o``).
  - Double check that the kernel still builds with the new CONFIG option turned
    off.