Update README.dart-sdk for inclusion of AOT snapshots

Updates the `snapshots/` description now that some Dart tools are included as AOT snapshots. Also updates the `utils/` description to be more generic since wasm-opt is bundled as well.

TEST=ci

Change-Id: If2611e9ae139ede772061ee62644e91d9b051555
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/332180
Reviewed-by: Michael Thomsen <[email protected]>
Auto-Submit: Parker Lougheed <[email protected]>
Reviewed-by: Siva Annamalai <[email protected]>
Commit-Queue: Michael Thomsen <[email protected]>

Author: parlough

README.dart-sdk

@@ -7,9 +7,9 @@ Here's a brief guide to what's in here:
 bin/               Binaries/scripts to compile, run, and manage Dart apps.
   dart             Command line Dart tool
   dartaotruntime   Minimal Dart runtime for running AOT modules
-  resources/       resource files for dartdoc and devtools
-  snapshots/       app-jit snapshots of various tools
-  utils/           tool for generating AOT snapshots
+  resources/       Resource files for dartdoc and devtools
+  snapshots/       AppAOT and AppJIT snapshots of various tools
+  utils/           Tools used by Dart compilers
 
 include/           header files that define the Dart embedding API for use by
                    - C/C++ applications that embed the Dart Virtual machine