[Clang][Docs] Fix malformed code-block directive in MSan and TSan docs (#190461)
The `code-block` directives in MemorySanitizer.rst and
ThreadSanitizer.rst were missing a leading period (`. code-block`
instead of `.. code-block`). This syntax error caused Sphinx to fail to
recognize the directives, resulting in the the subsequent C code being
rendered as plain text rather than a syntax-highlighted block.
The currently broken rendering on the official docs can be seen
[here](https://clang.llvm.org/docs/MemorySanitizer.html#interaction-of-inlining-with-disabling-sanitizer-instrumentation)
and
[here](https://clang.llvm.org/docs/ThreadSanitizer.html#interaction-of-inlining-with-disabling-sanitizer-instrumentation).
Fixed the typos to ensure proper HTML rendering.