content: fix search page header font-size
content: fix search page header font-size

A bug was introduced in commit 5cd22b744a8b4c5519f986f6f0432c8f1aec57a2,
when the font-size of SearchSnippet-header was inteaded to be changed to
1.5rem, but SearchResults-header was set instead.

Fixes b/137689269

Change-Id: Idc48cc8fb4a9e26fc5d991cbe99fc3d168372f20
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/507997
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite