chore(rstcheck): update config key name

The config for rstcheck changed slightly, it expects ignore_language to
be ignore_languages now.

Signed-off-by: Randolph Sapp <rs@ti.com>

.rstcheck.cfg

@@ -2,4 +2,4 @@
 ignore_directives = ifconfig
 # ignore the following languages because they require external compilers and
 # snippets in this repo depend on external code
-ignore_language = c,cpp,c++
+ignore_languages = c,cpp,c++