Fix: PBS127 fix typo

In the "NPM for Dependency Management" section, 6th paragraph, 1st sentence: "... a JavaScript version of the exiting ...", "exiting" s/b "existing".

docs/pbs127.md

@@ -60,7 +60,7 @@ For example, to import the latest version of the MomentJS module into a NodeJS p
 
 Almost all the 3rd-party code we've used so far in this series has been used as traditional dependencies, but there are two exceptions — the testing framework [QUnit](https://qunitjs.com/) is a dev dependency, as is the document generator [JSDoc](https://jsdoc.app/). QUnit helps us create code, but the code we create runs just fine without it. Our code does not depend on QUnit, but our development process does.
 
-Our first milestone is to ship a JavaScript version of the exiting [Crypt::HSXKPasswd](https://metacpan.org/pod/Crypt::HSXKPasswd) Perl module. The first thing we'll be using NPM for is to manage our dev dependencies, specifically, to provide us with:
+Our first milestone is to ship a JavaScript version of the existing [Crypt::HSXKPasswd](https://metacpan.org/pod/Crypt::HSXKPasswd) Perl module. The first thing we'll be using NPM for is to manage our dev dependencies, specifically, to provide us with:
 
 1. A code linter [ESLint](https://eslint.org/)
 2. A testing framework (TBD)