Fix 22643 - Readme links to vanished file (#22647)

* Fixed dead link to browser list

The link to the list of browsers supported through Autoprefixer was dead, now updated to a working link.

* Updated label to display new file path

Changed the labeling on the updated link to show read as the new updated file path.

docs/getting-started/build-tools.md

@@ -40,7 +40,7 @@ Our [package.json](https://github.com/twbs/bootstrap/blob/master/package.json) i
 
 Bootstrap uses [Autoprefixer][autoprefixer] (included in our build process) to automatically add vendor prefixes to some CSS properties at build time. Doing so saves us time and code by allowing us to write key parts of our CSS a single time while eliminating the need for vendor mixins like those found in v3.
 
-We maintain the list of browsers supported through Autoprefixer in a separate file within our GitHub repository. See [`/build/postcss.js`](https://github.com/twbs/bootstrap/blob/v4-dev/build/postcss.js) for details.
+We maintain the list of browsers supported through Autoprefixer in a separate file within our GitHub repository. See [`/build/postcss.config.js`](https://github.com/twbs/bootstrap/blob/v4-dev/build/postcss.config.js) for details.
 
 ## Local documentation