Fix typos in gen_snapshot usage.

Closes #30919

Change-Id: I30ce344914c4a72ce340a06a0c7fad4a6b037781
Reviewed-on: https://dart-review.googlesource.com/9375
Reviewed-by: Kevin Moore <kevmoo@google.com>

Author: lexaknyazev

runtime/bin/gen_snapshot.cc

@@ -752,8 +752,7 @@ static void PrintUsage() {
 "                                                                            \n"
 " To create an AOT application snapshot as assembly suitable for compilation \n"
 " as a static or dynamic library:                                            \n"
-" mmap:                                                                      \n"
-"   --snapshot_kind=app-aot-blobs                                            \n"
+"   --snapshot_kind=app-aot-assembly                                         \n"
 "   --assembly=<output-file>                                                 \n"
 "   {--embedder_entry_points_manifest=<input-file>}                          \n"
 "   [--obfuscate]                                                            \n"