Fix duplicate 'the' on "Creating packages" page (#4648)

Delete one 'the' in sentence "When ..."

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Co-authored-by: Anthony Sansone <atsansone@users.noreply.github.com>
Co-authored-by: Parker Lougheed <parlough@gmail.com>

Author: 3 people

src/_guides/libraries/create-library-packages.md

@@ -121,7 +121,7 @@ This adapter handles HttpRequest objects from `dart:io`.
 
 ## Importing library files
 
-When importing a library file from another package, use the
+When importing a library file from another package, use
 the `package:` directive to specify the URI of that file.
 
 ```dart