[x/blog] content: correct mistake in "Slices" article
[x/blog] content: correct mistake in "Slices" article

Change-Id: Ie994d952ff94f05c63fc63389ec359d3e9a902e9
Reviewed-on: https://go-review.googlesource.com/64590
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
X-Blog-Commit: 1caafe26a32b5423a75df95e4c3e9ccd2c3a27d3

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website