docs: contribute: documentation: generation: fix missing doc

fix the path where the user has to be in order to execute make html-fast

(cherry picked from commit 5f418f5)

Original-Signed-off-by: Jilay Pandya <jilay.pandya@outlook.com>
GitOrigin-RevId: 5f418f5
Cr-Build-Id: 8731209674273728625
Cr-Build-Url: https://cr-buildbucket.appspot.com/build/8731209674273728625
Copybot-Job-Name: zephyr-main-copybot-downstream
Change-Id: I4a9dc1edbd1a0c94dbc284ed1817c63a7f5e07e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/6025249
Tested-by: Al Semjonovs <asemjonovs@google.com>
Commit-Queue: Al Semjonovs <asemjonovs@google.com>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Reviewed-by: Al Semjonovs <asemjonovs@google.com>

doc/contribute/documentation/generation.rst

@@ -255,7 +255,7 @@ To enable this mode, set the following option when invoking cmake::
 
 or invoke make with the following target::
 
-   cd ~/zephyr
+   cd ~/zephyr/doc
 
    # To generate HTML output without detailed Kconfig
    make html-fast