cmd/go: fix retract interval syntax in 'go help mod edit'

For #24031

Change-Id: I70461431aac24c9465b9bdab082bcc34343a53a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/270557
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>

Author: Jay Conrod

src/cmd/go/alldocs.go

@@ -69,7 +69,7 @@ a version on the left side is dropped.
 
 The -retract=version and -dropretract=version flags add and drop a
 retraction on the given version. The version may be a single version
-like "v1.2.3" or a closed interval like "[v1.1.0-v1.1.9]". Note that
+like "v1.2.3" or a closed interval like "[v1.1.0,v1.1.9]". Note that
 -retract=version is a no-op if that retraction already exists.
 
 The -require, -droprequire, -exclude, -dropexclude, -replace,