fix help for the --print option

Since 8285ab5, which was merged in with #38061, the help for the
--print option is missing the surrounding [ ] around the possible
options.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>

src/librustc/session/config.rs

@@ -1182,7 +1182,8 @@ pub fn rustc_short_optgroups() -> Vec<RustcOptGroup> {
                               the compiler to emit",
                  "[asm|llvm-bc|llvm-ir|obj|metadata|link|dep-info]"),
         opt::multi_s("", "print", "Comma separated list of compiler information to \
-                               print on stdout", &print_opts.join("|")),
+                               print on stdout", &format!("[{}]",
+                               &print_opts.join("|"))),
         opt::flagmulti_s("g",  "",  "Equivalent to -C debuginfo=2"),
         opt::flagmulti_s("O", "", "Equivalent to -C opt-level=2"),
         opt::opt_s("o", "", "Write output to <filename>", "FILENAME"),