doc/go1.20: correct test binary -v flag value for test2json

The -v flag value is "test2json", not "json", since it emits output
in a custom format that the cmd/test2json tool interprets.
The cmd/test2json documentation and implementation have this right.

For #54202.

Change-Id: I2b52861d926e14488aa9fc89fff8c26da32ca710
Reviewed-on: https://go-review.googlesource.com/c/go/+/456124
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

doc/go1.20.html

@@ -102,9 +102,9 @@ <h3 id="go-command">Go command</h3>
   Programs that run <code>go</code> <code>test</code> <code>-json</code>
   do not need any updates.
   Programs that invoke <code>go</code> <code>tool</code> <code>test2json</code>
-  directly should now run the test binary with <code>-v=json</code>
-  (for example, <code>go</code> <code>test</code> <code>-v=json</code>
-  or <code>./pkg.test</code> <code>-test.v=json</code>)
+  directly should now run the test binary with <code>-v=test2json</code>
+  (for example, <code>go</code> <code>test</code> <code>-v=test2json</code>
+  or <code>./pkg.test</code> <code>-test.v=test2json</code>)
   instead of plain <code>-v</code>.
 </p>