blog/_content/gouk15: add image width to embedded jpg

Change-Id: I38496029cecde29e6358bacc49cfe1e1ec28fd18
Reviewed-on: https://go-review.googlesource.com/c/website/+/342090
Trust: Russ Cox <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Emmanuel Odeke <[email protected]>

Author: rsc

blog/_content/gouk15.article

@@ -11,7 +11,7 @@ On August 21st the Go community gathered in London for the first edition of
 [Golang UK](https://golanguk.com). The conference featured two parallel
 tracks and nearly 400 gophers attended.
 
-.image gouk15/gouk.jpg 300 _
+.image gouk15/gouk.jpg 300 583
 
 The conference started with the opening keynote by [David Calavera](https://twitter.com/calavera)
 called Crossing the Language Chasm ([video](https://www.youtube.com/watch?v=JPVRnEZ4v_w&list=PLDWZ5uzn69ezRJYeWxYNRMYebvf8DerHd))