fix error in metric tracked statement

[konfirmed]

I'd like to make sure that the text is corrected.

correcting the number of metrics tracked in the Performance section of the lighthouse report

Changes proposed in this pull request:

• The number of metrics tracked changed,

[chrome-devrel-infra[bot]]

:white_check_mark: Preview (static) for commit 71d1a789620fb8a729f56584cc5f42c6f5868342 available at https://pr-7696-static-dot-dcc-staging.uc.r.appspot.com/.

The following pages likely changed with this PR:

• https://pr-7696-static-dot-dcc-staging.uc.r.appspot.com/docs/lighthouse/performance/first-contentful-paint/