[x/blog] g.blog/slices: trivial fix to PtrSubtractOneFromLength example

There should be a := not an = in the body of the function.
Another broken example working by accident.

R=adg
CC=golang-dev
https://golang.org/cl/14059044
X-Blog-Commit: 4f303a1af2796bdcdf14fea5d4974f6c3b26c407

blog/content/slices/prog030.go

@@ -12,7 +12,7 @@ var buffer [256]byte
 var slice []byte = buffer[100:150]
 
 func PtrSubtractOneFromLength(slicePtr *[]byte) {
-	slice = *slicePtr
+	slice := *slicePtr
 	*slicePtr = slice[0 : len(slice)-1]
 }