test/fixedbugs: Close opening parenthesis in comment in issue20789.go

Updates #20789

Change-Id: Ic7a94394e283bc1b667cdad999ceee9513f35e37
Reviewed-on: https://go-review.googlesource.com/46770Reviewed-by: Robert Griesemer <gri@golang.org>

test/fixedbugs/issue20789.go

@@ -7,7 +7,7 @@
 // Make sure this doesn't crash the compiler.
 // (This test should be part of the internal/syntax
 // package, but we don't have a general test framework
-// there yet, so put it here for now. See also #20800.
+// there yet, so put it here for now. See also #20800.)
 
 package e
 func([<-chan<-[func u){go // ERROR "unexpected u", ERROR "must be function call"
\ No newline at end of file