test-manual: Correct two references to BB_SKIP_NETTESTS

The environment variable used to disable network tests when running
bitbake-selftest was incorrectly referred to as BB_SKIP_NETTEST.

(From yocto-docs rev: 4a2252cf412ea2aa26209d835c96661a772b6d31)

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Reviewed-by: Quentin Schulz <foss+yocto@0leil.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/test-manual/intro.rst

@@ -178,10 +178,10 @@ Tests map into the codebase as follows:
 
       $ bitbake-selftest
 
-   To skip tests that access the Internet, use the ``BB_SKIP_NETTEST``
+   To skip tests that access the Internet, use the ``BB_SKIP_NETTESTS``
    variable when running "bitbake-selftest" as follows::
 
-      $ BB_SKIP_NETTEST=yes bitbake-selftest
+      $ BB_SKIP_NETTESTS=yes bitbake-selftest
 
    The default output is quiet and just prints a summary of what was
    run. To see more information, there is a verbose option::