Small fix for Bazel instructions. (#875)

In the past the instructions were to copy `emscripten_toolchain` into the project dir, now it downloads emsdk as a package so the `bazelrc` part has to refer to `@emsdk`.

bazel/README.md

diff --git a/bazel/README.md b/bazel/README.md
index dc849fd..b7b3ce5 100644
--- a/bazel/README.md
+++ b/bazel/README.md
@@ -25,7 +25,7 @@
 
 Put the following lines into your `.bazelrc`:
 ```
-build:wasm --crosstool_top=//emscripten_toolchain:everything
+build:wasm --crosstool_top=@emsdk//emscripten_toolchain:everything
 build:wasm --cpu=wasm
 build:wasm --host_crosstool_top=@bazel_tools//tools/cpp:toolchain
 ```