harfbuzz-ng: .gitignore the correct directory

Followup to https://chromium-review.googlesource.com/c/chromium/src/+/970242

Bug: 814305
Change-Id: I3e034eb8c1bc093cc6117641c625059f0e208f70
Reviewed-on: https://chromium-review.googlesource.com/970471Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544337}

third_party/.gitignore

@@ -85,7 +85,7 @@
 /gperf
 /guava/lib/
 /hamcrest/lib/
-/harfbuzz/src
+/harfbuzz-ng/src
 /hunspell_dictionaries
 /icu
 /icu4j/lib/