[piex-wasm] README.md: correct the wasm binary name

CL:1596788 mentioned the binary code was called piex.out.wasm, but the
README.md does not say that. Make it so (comment only change).

Tbr: adanilo@
No-try: true
Bug: 935285
Change-Id: Iffe0957b410a2797ed14afc0b68d6fb08442fa30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611698
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659451}

ui/file_manager/image_loader/piex/README.md

@@ -15,7 +15,7 @@ Install piexwasm project components
   npm install
 ```
 
-Build piexwasm code: piex.js.wasm piex.wasm
+Build piexwasm code: piex.js.wasm piex.out.wasm
 
 ```shell
   npm run build
@@ -27,7 +27,7 @@ Run tests: they must PASS
   npm run test
 ```
 
-Release: submit piex.js.wasm piex.wasm to the Chromium repository
+Release: submit piex.js.wasm piex.out.wasm to the Chromium repository
 
 ```shell
   git commit -a -m "Release piexwasm ..."