[NFC] Inclusive language: replace master with main in benchmark docs

[quinnp]

Summary

[NFC] As part of using inclusive language within the llvm project and to match
the renamed master branch of google/benchmark, this patch replaces master with
main in the benchmark releasing docs.

llvm/utils/benchmark/releasing.md

 # How to release
 
-* Make sure you're on master and synced to HEAD
+* Make sure you're on main and synced to HEAD
 * Ensure the project builds and tests run (sanity check only, obviously)
     * `parallel -j0 exec ::: test/*_test` can help ensure everything at least
       passes
 * Prepare release notes
     * `git log $(git describe --abbrev=0 --tags)..HEAD` gives you the list of
       commits between the last annotated tag and HEAD
     * Pick the most interesting.
 * Create a release through github's interface
     * Note this will create a lightweight tag.
     * Update this to an annotated tag:
       * `git pull --tags`
       * `git tag -a -f <tag> <tag>`
       * `git push --force origin`