Point efficient_fuzzer.md to https://llvm.org.

testing/libfuzzer/efficient_fuzzer.md

 # Efficient Fuzzer
 
 This document describes ways to determine your fuzzer efficiency and ways 
 to improve it.
 
 ## Overview
 
 Being a coverage-driven fuzzer, libFuzzer considers a certain input *interesting*
 if it results in new coverage. The set of all interesting inputs is called 
 *corpus*. 
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 You can also access source-level coverage report locally:
 
 ```bash
 # produces binary .sancov file
 ASAN_OPTIONS=coverage=1 ./out/libfuzzer/my_fuzzer -runs=0 ~/tmp/my_fuzzer_corpus
 # Convert binary .sancov to symbolized .symcov file.
 ./third_party/llvm-build/Release+Asserts/bin/sancov \
   -symbolize my_fuzzer my_fuzzer.123.sancov > my_fuzzer.symcov
 # Launch coverage report server
-curl http://llvm.org/svn/llvm-project/llvm/trunk/tools/sancov/coverage-report-server.py | python3 \
+curl https://llvm.org/svn/llvm-project/llvm/trunk/tools/sancov/coverage-report-server.py | python3 \
   --symcov my_fuzzer.symcov --srcpath path_to_chromium_sources
 # Navigate to http://localhost:8001/ to view coverage report
 ```
 Replace `ASAN_OPTIONS` by corresponding option variable if your are using 
 another sanitizer (e.g. `MSAN_OPTIONS`).
 
 *NOTE: This is an experimental feature and an active area of work. We are 
 working on improving this process.*
 
 
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 ```
 
 Please note that `dict` parameter should be provided [separately](#Fuzzer-Dictionary).
 Other options may be passed through `libfuzzer_options` property.
 
 
 [AFL]: http://lcamtuf.coredump.cx/afl/
 [ClusterFuzz status]: clusterfuzz.md#Status-Links
 [Corpus GCS Bucket]: https://goto.google.com/libfuzzer-clusterfuzz-corpus
 [issue 638836]: https://bugs.chromium.org/p/chromium/issues/detail?id=638836