[ruby/optparse] Use ZDOTDIR env var to locate .zshrc

https://github.com/ruby/optparse/commit/c4977674bf

misc/rb_optparse.zsh

@@ -7,9 +7,9 @@
 #      cp rb_optparse.zsh ~/.zsh.d/rb_optparse.zsh
 #
 # (2) load the script, and add a directory to fpath before compinit.
-#      echo '. ~/.zsh.d/rb_optparse.zsh' >> ~/.zshrc
-#      echo 'fpath=(~/.zsh.d/Completion $fpath)' >> ~/.zshrc
-#      echo 'autoload -U compinit; compinit' >> ~/.zshrc
+#      echo '. ~/.zsh.d/rb_optparse.zsh' >> "${ZDOTDIR:-~}/.zshrc"
+#      echo 'fpath=(~/.zsh.d/Completion $fpath)' >> "${ZDOTDIR:-~}/.zshrc"
+#      echo 'autoload -U compinit; compinit' >> "${ZDOTDIR:-~}/.zshrc"
 #
 # (3) restart zsh.
 #