Capitalize Package in a Heading

The third heading needed “package” capitalized.

PR-URL: #2749
Credit: @MrBrain295
Close: #2749
Reviewed-by: @isaacs

Author: MrBrain295

docs/content/using-npm/developers.md

@@ -20,7 +20,7 @@ These are man pages.  If you install npm, you should be able to
 then do `man npm-thing` to get the documentation on a particular
 topic, or `npm help thing` to see the same information.
 
-### What is a package
+### What is a Package
 
 A package is: