brew doctorでエラーが出る

###前提・実現したいこと
Homebrewのmysqlがなぜか突然動かなくなり、homebrewの環境を整理しようと思った際に起こった問題です。

brew --configの結果

HOMEBREW_VERSION: 0.9.9
ORIGIN: https://github.com/Homebrew/brew.git
HEAD: fdf55e77e10f938d1c5d8a72e015e04451be938a
Last commit: 35 minutes ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: d7144eb7a1919497351aa2ef775f4b0a7c7edb48
Core tap last commit: 29 minutes ago
HOMEBREW_PREFIX: /usr/local
HOMEBREW_REPOSITORY: /usr/local
HOMEBREW_CELLAR: /usr/local/Cellar
HOMEBREW_BOTTLE_DOMAIN: https://homebrew.bintray.com
CPU: quad-core 64-bit haswell
Homebrew Ruby: 2.0.0-p648
Clang: 7.3 build 703
Git: 2.9.0 => /usr/local/bin/git
Perl: /usr/bin/perl
Python: /usr/bin/python
Ruby: /Users/username/.rbenv/shims/ruby => /Users/username/.rbenv/versions/2.3.0/bin/ruby
Java: N/A
OS X: 10.11.6-x86_64
Xcode: 7.3.1
CLT: 7.3.1.0.1.1461711523

###発生している問題・エラーメッセージ

Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry and just ignore them. Thanks!

Warning: Homebrew's sbin was not found in your PATH but you have installed
formulae that put executables in /usr/local/sbin.
Consider setting the PATH for example like so
  echo 'export PATH="/usr/local/sbin:$PATH"' >> ~/.zshrc

###試したこと

エラーメッセージの通りにPATHを書き換えてもうまくいきませんでした。