[libcxx][docs] Fix bullet point in Additional Tools section

Without a blank line after the ":", it was rendered on the same
line instead of a new one.

Author: DavidSpickett

libcxx/docs/TestingLibcxx.rst

@@ -116,6 +116,7 @@ Additional tools
 The libc++ test suite uses a few optional tools to improve the code quality.
 
 These tools are:
+
 - clang-tidy (you might need additional dev packages to compile libc++-specific clang-tidy checks)
 
 Reproducing CI issues locally