actually test rustc-guide with toolstate

Author: mark-i-m

src/ci/docker/x86_64-gnu-tools/checktools.sh

@@ -25,6 +25,7 @@ python2.7 "$X_PY" test --no-fail-fast \
     src/doc/rust-by-example \
     src/doc/embedded-book \
     src/doc/edition-guide \
+    src/doc/rustc-guide \
     src/tools/clippy \
     src/tools/rls \
     src/tools/rustfmt \