spec: improve the example in Type assertions section

The example, var v, ok T1 = x.(T), can be interpreted as type T1 interface{} or type T = bool; type T1 = T.
Separating the example would help understanding for readers.

Change-Id: I179f4564e67f4d503815d29307df2cebb50c82f9
GitHub-Last-Rev: b34fffb6bb07cb2883bc313ef3bc9980b3dd4abe
GitHub-Pull-Request: golang/go#44040
Reviewed-on: https://go-review.googlesource.com/c/go/+/288472
Reviewed-by: Robert Griesemer <[email protected]>
Reviewed-by: Rob Pike <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Trust: Robert Griesemer <[email protected]>

Author: task4233

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of Oct 7, 2020",
+	"Subtitle": "Version of Feb 2, 2021",
 	"Path": "/ref/spec"
 }-->
 
@@ -3400,7 +3400,7 @@ <h3 id="Type_assertions">Type assertions</h3>
 v, ok = x.(T)
 v, ok := x.(T)
 var v, ok = x.(T)
-var v, ok T1 = x.(T)
+var v, ok interface{} = x.(T) // dynamic types of v and ok are T and bool
 </pre>
 
 <p>