Update README.md

Co-Authored-By: Ralf Jung <post@ralfj.de>

README.md

@@ -171,7 +171,7 @@ Several `-Z` flags are relevant for Miri:
   sets this flag per default.
 * `-Zmir-emit-retag` controls whether `Retag` statements are emitted. Miri
   enables this per default because it is needed for validation.
-* `-Zmiri-track-pointer-tag` aborts interpretation with a backtrace when the
+* `-Zmiri-track-pointer-tag=<tag>` aborts interpretation with a backtrace when the
   given pointer tag is popped from a borrow stack (which is where the tag
   becomes invalid and any future use of it will error anyway).  This helps you
   in finding out why UB is happening and where in your code would be a good