content: change "POPULAR SEARCHES" to "EXAMPLE SEARCHES"
content: change "POPULAR SEARCHES" to "EXAMPLE SEARCHES"

The searches listed on the homepage are example searches, not
necessarily the most popular. The caption is changed accordingly.

Change-Id: Ife71e2b8856b45526a5cd88c2214c0bd06dbf6af
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/246439
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

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