Update MemorySanitizerLibcxxHowTo build script

Given https://reviews.llvm.org/D132480 the script provided at [MemorySanitizerLibcxxHowTo](https://github.com/google/sanitizers/wiki/MemorySanitizerLibcxxHowTo) is broken.

I believe it should be fixed like this:
```
-	-DLLVM_ENABLE_PROJECTS="libcxx;libcxxabi" \
+	-DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi" \
```