[lit] Fix typo.
 - Noticed by edward-san (IRC).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188096 91177308-0d34-0410-b5e6-96231b3b80d8
[docs] Update TestingGuide's note on how to run with Valgrind.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188097 91177308-0d34-0410-b5e6-96231b3b80d8