Second attempt to fix Sphinx bot

The fix in r282148 was not enough to fix the following error:
/home/llvmbb/llvm-build-dir/clang-sphinx-docs/llvm/src/tools/clang/docs/CommandGuide/clang.rst:338:
WARNING: unknown option: -flto=full

on the sphinx bot:
  http://lab.llvm.org:8011/builders/clang-sphinx-docs/builds/16313

(not reproducible locally).

This time, simply remove the option reference.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@282151 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CommandGuide/clang.rst

@@ -335,7 +335,7 @@ Code Generation Options
   assembly files, otherwise this generates LLVM bitcode format object files
   (which may be passed to the linker depending on the stage selection options).
 
-  The default for :option:`-flto` is :option:`-flto=full`, in which the
+  The default for :option:`-flto` is "full", in which the
   LLVM bitcode is suitable for monolithic Link Time Optimization (LTO), where
   the linker merges all such modules into a single combined module for
   optimization. With :option:`-flto=thin`, :doc:`ThinLTO <../ThinLTO>`