docs: Fix code blocks in toolchains docs

Signed-off-by: Andrei Gherzan andrei.gherzan@huawei.com

docs/toolchains.rst

@@ -23,6 +23,7 @@ gcc
 Post-2.0 releases of |main_project_name| will likely update to new major versions.
 
 To select gcc, edit `conf/local.conf` and add:
+
 .. code-block:: sh
 
    TOOLCHAIN="gcc"
@@ -35,11 +36,13 @@ clang
 Post-2.0 releases of |main_project_name| will likely update to new major versions.
 
 To select clang, edit `conf/local.conf` and add:
+
 .. code-block:: sh
 
    TOOLCHAIN="clang"
 
 You may also want to add:
+
 .. code-block:: sh
 
    RUNTIME="llvm"