[docs][llvm-ar] Fix option:: O after r375106

docs-llvm-html fails => unknown option: O

There are lots of formatting issues in the file but they will be fixed by D68998.

llvm-svn: 375107
diff --git a/llvm/docs/CommandGuide/llvm-ar.rst b/llvm/docs/CommandGuide/llvm-ar.rst
index a138dcf..8a36a40 100644
--- a/llvm/docs/CommandGuide/llvm-ar.rst
+++ b/llvm/docs/CommandGuide/llvm-ar.rst
@@ -145,7 +145,7 @@
  When extracting files, this option will cause **llvm-ar** to preserve the
  original modification times of the files it writes.
 
-[O]
+.. option:: O
 
  Display member offsets inside the archive.