[GN docs] Add missing word in gn actions help string

gn actions help said "run a script a single time to produce or more
output files". Add the missing "one" to make this "one or more output
files".

BUG=None

Review-Url: https://codereview.chromium.org/2133873002
Cr-Original-Commit-Position: refs/heads/master@{#404533}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: bf462ea0de8604630aecc49e976c3bd9d646a4a0
2 files changed
tree: fbf083d4814e9b751abd6bc91a437b06367b3be9
  1. tools/