fix docstring of open(command) (close JuliaLang#24649)

base/process.jl

@@ -574,9 +574,9 @@ end
 """
     open(command, stdio=devnull; write::Bool = false, read::Bool = !write)
 
-Start running `command` asynchronously, and return a tuple `(stream,process)`.  If `read` is
-true, then `stream` reads from the process's standard output and `stdio` optionally
-specifies the process's standard input stream.  If `write` is true, then `stream` writes to
+Start running `command` asynchronously, and return a `process` object.  If `read` is
+true, then `process` reads from the process's standard output and `stdio` optionally
+specifies the process's standard input stream.  If `write` is true, then `process` writes to
 the process's standard input and `stdio` optionally specifies the process's standard output
 stream.
 """