docs: fix new shims contributing typo

[MikeMcC399]

Issue

Adding a new package manager contains text with repeated words / phrases:

• package.json to add to add the added shims to the list of
  "publishConfig/bin" and "executableFiles",

with add / add / added

Change to

• package.json to add new shims to the list of
  "publishConfig/bin" and "executableFiles",