docs(readme): update sed script for BSD compat

Author: Ryan Sonshine

README.md

@@ -37,7 +37,8 @@ Replace `FULL_NAME`, `GITHUB_USER`, and `REPO_NAME` in the script below with you
 FULL_NAME="John Smith"
 GITHUB_USER="johnsmith"
 REPO_NAME="my-cool-package"
-sed -i "s/ryansonshine/$GITHUB_USER/g; s/typescript-npm-package-template\|my-package-name/$REPO_NAME/g; s/Ryan Sonshine/$FULL_NAME/g" package.json package-lock.json README.md
+sed -i.mybak "s/ryansonshine/$GITHUB_USER/g; s/typescript-npm-package-template\|my-package-name/$REPO_NAME/g; s/Ryan Sonshine/$FULL_NAME/g" package.json package-lock.json README.md
+rm *.mybak
 ```
 
 ### Add NPM Token