internal/lsp/cmd: include new name in rename help message

"gopls rename" appears to require a new name as a positional argument
but the command line help message doesn't include it. Update the help
message.

Change-Id: Iea769d2095b0642ebbacce128da336ec0dc56828
Reviewed-on: https://go-review.googlesource.com/c/tools/+/280472
Reviewed-by: Rebecca Stambler <[email protected]>
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>
Trust: Rebecca Stambler <[email protected]>
Trust: Hyang-Ah Hana Kim <[email protected]>
Run-TryBot: Rebecca Stambler <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: ericchiang

internal/lsp/cmd/rename.go

@@ -31,15 +31,15 @@ type rename struct {
 }
 
 func (r *rename) Name() string      { return "rename" }
-func (r *rename) Usage() string     { return "<position>" }
+func (r *rename) Usage() string     { return "<position> <new name>" }
 func (r *rename) ShortHelp() string { return "rename selected identifier" }
 func (r *rename) DetailedHelp(f *flag.FlagSet) {
 	fmt.Fprint(f.Output(), `
 Example:
 
   $ # 1-based location (:line:column or :#position) of the thing to change
-  $ gopls rename helper/helper.go:8:6
-  $ gopls rename helper/helper.go:#53
+  $ gopls rename helper/helper.go:8:6 Foo
+  $ gopls rename helper/helper.go:#53 Foo
 
 	gopls rename flags are:
 `)