doc: add the valid link for curl(1) in repl.md

The current autoinserted link leads to 404 page.

PR-URL: https://github.com/nodejs/node/pull/10244
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/repl.md

@@ -85,7 +85,7 @@ undefined
 ```
 
 Unless otherwise scoped within blocks or functions, variables declared
-either implicitly, or using the `const`, `let`, or `var` keywords
+either implicitly or using the `const`, `let`, or `var` keywords
 are declared at the global scope.
 
 #### Global and Local Scope
@@ -540,10 +540,11 @@ possible to connect to a long-running Node.js process without restarting it.
 For an example of running a "full-featured" (`terminal`) REPL over
 a `net.Server` and `net.Socket` instance, see: https://gist.github.com/2209310
 
-For an example of running a REPL instance over curl(1),
+For an example of running a REPL instance over [curl(1)][],
 see: https://gist.github.com/2053342
 
 [stream]: stream.html
 [`util.inspect()`]: util.html#util_util_inspect_object_options
 [`readline.Interface`]: readline.html#readline_class_interface
 [`readline.InterfaceCompleter`]: readline.html#readline_use_of_the_completer_function
+[curl(1)]: https://curl.haxx.se/docs/manpage.html