[docs] Fix a sphinx error in llvm-cov.rst

Failing bot:

  http://lab.llvm.org:8011/builders/llvm-sphinx-docs/builds/12025

Fix tested with `ninja docs-llvm-html`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@276820 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CommandGuide/llvm-cov.rst

@@ -317,6 +317,10 @@ OPTIONS
  universal binary or to use an architecture that does not match a
  non-universal binary.
 
+.. program:: llvm-cov export
+
+.. _llvm-cov-export:
+
 EXPORT COMMAND
 --------------