Tell users to use inline-8bit-counters.

libfuzzer defaults to `trace-pc-guard`, but that seems to be removed
now:

- #66
- llvm/llvm-project@c5d3d49#diff-32378db1cb0e8e11ecfc62eda7181acadc661910d4e3ab214babb65af95bc7e9

And confusingly, their docs still mention `trace-pc-guard`:

- https://github.com/llvm/llvm-project/blob/master/clang/docs/SanitizerCoverage.rst

Author: frewsxcv

README.md

@@ -51,6 +51,7 @@ Build by running the following command:
 $ cargo rustc -- \
     -C passes='sancov' \
     -C llvm-args='-sanitizer-coverage-level=3' \
+    -C llvm-args='-sanitizer-coverage-inline-8bit-counters' \
     -Z sanitizer=address
 ```