Fix ThreadLocalUsage documentation formatting.

Fixes #897

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181668862

docs/bugpattern/ThreadLocalUsage.md

@@ -1,8 +1,8 @@
 `ThreadLocal`s should be stored in `static` variables to avoid memory leaks. If
-a `ThreadLocal` is stored in an instance (non-static) variable, there will be `M
-* N` instances of the `ThreadLocal` value where `M` is the number of threads,
-and `N` is the number of instances of the containing class. Each instance may
-remain live as long the thread that stored it stays live.
+a `ThreadLocal` is stored in an instance (non-static) variable, there will be
+<code>M \* N</code> instances of the `ThreadLocal` value where `M` is the number
+of threads, and `N` is the number of instances of the containing class. Each
+instance may remain live as long the thread that stored it stays live.
 
 Example: