[CrOS] Fix instructions for espeak-ng installation on Linux

Previous instructions in 2530745 installed the whole espeak-ng tarball,
and not just the chrome-extension/ directory, under
/usr/share/chromeos-assets/speech_synthesis/espeak-ng .

Moreover attempt to provide instructions which would work on standard
linux / rodete to create the directory and set the permissions correctly.

R=dmazzoni@chromium.org, dtseng@chromium.org
Signed-off-by: Guido Trotter <ultrotter@chromium.org>
Change-Id: I41193bf02374f0bdde55371619ed4db72dae63e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536434Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827571}

docs/accessibility/chromevox_on_desktop_linux.md

@@ -85,9 +85,13 @@ gsutil ls gs://chromeos-localmirror/distfiles/espeak*
 Pick the latest version and
 
 ```
-gsutil cp gs://chromeos-localmirror/distfiles/espeak-ng-20180801.tar.gz /usr/share/chromeos-assets/speech_synthesis/espeak-ng/
-tar xvf /usr/share/chromeos-assets/speech_synthesis/espeak-ng/espeak-ng-20180801.tar.gz
-rm /usr/share/chromeos-assets/speech_synthesis/espeak-ng/espeak-ng-20180801.tar.gz
+TMPDIR=$(mktemp -d)
+gsutil cp gs://chromeos-localmirror/distfiles/espeak-ng-20180801.tar.gz $TMPDIR
+tar -C $TMPDIR -xvf ~/espeak-ng/espeak-ng-20180801.tar.gz
+sudo mkdir -p /usr/share/chromeos-assets/speech_synthesis/espeak-ng/
+sudo chown -R $(whoami) /usr/share/chromeos-assets/
+cp -r $TMPDIR/espeak-ng/chrome-extension/* /usr/share/chromeos-assets/speech_synthesis/espeak-ng
+rm -rf $TMPDIR
 ```
 
 **Be sure to check permissions of /usr/share/chromeos-assets, some users report