[docs] Coverage: Reference the new -output-dir option

llvm-svn: 277081
diff --git a/clang/docs/SourceBasedCodeCoverage.rst b/clang/docs/SourceBasedCodeCoverage.rst
index b56675d..8c4ab8d 100644
--- a/clang/docs/SourceBasedCodeCoverage.rst
+++ b/clang/docs/SourceBasedCodeCoverage.rst
@@ -115,7 +115,7 @@
 
 .. code-block:: console
 
-    % llvm-cov show ./foo -instr-profile=foo.profdata -format html -Xdemangler c++filt -Xdemangler -n
+    % llvm-cov show ./foo -instr-profile=foo.profdata -format html -o report.dir -Xdemangler c++filt -Xdemangler -n
 
 This report includes a summary view as well as dedicated sub-views for
 templated functions and their instantiations. For our example program, we get