win_cross: fix reference to GYP_MSVS_HASH_<toolchain hash> variable

The documentation had a typo in the variable mapping the hashes.

Bug: 852347
Change-Id: If69581b2bec94698208f7cf9f3d91b63e8048733
Reviewed-on: https://chromium-review.googlesource.com/1152908Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579018}

docs/win_cross.md

@@ -58,7 +58,7 @@ generated file in a Linux or Mac host, the following environment variables
 need to be set:
 
     export DEPOT_TOOLS_WIN_TOOLCHAIN_BASE_URL=<path/to/sdk/zip/file>
-    export GYP_MSVS_<toolchain hash>=<hash value>
+    export GYP_MSVS_HASH_<toolchain hash>=<hash value>
 
 `<toolchain hash>` is hardcoded in `src/build/vs_toolchain.py` and can be found by
 setting `DEPOT_TOOLS_WIN_TOOLCHAIN_BASE_URL` and running `gclient sync`: