doc/mem: remove unnecessary pre tags within same snippet

currently the snippet is segmented but should be one code snippet.

Change-Id: Ic747faf9bb1b52f9d1786eca70616a05b71ee801
Reviewed-on: https://go-review.googlesource.com/c/go/+/211198
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

doc/go_mem.html

@@ -273,9 +273,7 @@ func f() {
 	a = "hello, world"
 	<-c
 }
-</pre>
 
-<pre>
 func main() {
 	go f()
 	c &lt;- 0