lit: Fix formatting in README.rst

This was preventing lit from being uploaded to pypi.

Reviewed By: mgorny

Differential Revision: https://reviews.llvm.org/D143419

(cherry picked from commit e6d670d)

Author: tstellar

llvm/utils/lit/README.rst

@@ -41,7 +41,7 @@ Please browse the issues labeled *tools:llvm-lit* in LLVM's issue tracker for
 ideas on what to work on:
 https://github.com/llvm/llvm-project/labels/tools%3Allvm-lit
 
-Before submitting patches, run the test suite to ensure nothing has regressed:
+Before submitting patches, run the test suite to ensure nothing has regressed::
 
     # From within your LLVM source directory.
     utils/lit/lit.py \