Docs: tweak code-block spacing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195049 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/WritingAnLLVMBackend.rst b/docs/WritingAnLLVMBackend.rst
index 35c853e..35a2d16 100644
--- a/docs/WritingAnLLVMBackend.rst
+++ b/docs/WritingAnLLVMBackend.rst
@@ -976,6 +976,7 @@
 This results in:
 
 .. code-block:: c++
+
   namespace X86 {
   namespace OpTypes {
   enum OperandType {