x/website: correct a small grammatical error at effective_go page
x/website: correct a small grammatical error at effective_go page

Change-Id: Ia58d129bb7d71adad62d77a8347392ea11d8c9e0
GitHub-Last-Rev: f085d0163304b8762a93ee3ffb66f511b00a6b96
GitHub-Pull-Request: golang/website#75
Reviewed-on: https://go-review.googlesource.com/c/website/+/336869
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Trust: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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