manual: fix test-pkg missing.config file name

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/adding-packages-tips.txt

@@ -105,7 +105,7 @@ The results mean:
   snippet were not present in the final configuration. This is due to
   options having dependencies not satisfied by the toolchain, such as
   for example a package that +depends on BR2_USE_MMU+ with a noMMU
-  toolchain. The missing options are reported in +config.missing+ in
+  toolchain. The missing options are reported in +missing.config+ in
   the output build directory (+~/br-test-pkg/TOOLCHAIN_NAME/+ by
   default).
 * `FAILED`: the build failed. Inspect the +logfile+ file in the output