Update using dev build README.md to correctly point to gzipped wasm

[letmejustputthishere]

the current releases end in gz, if we're not providing the correct file ending dfx fails

❯ dfx deploy
Deploying all canisters.
Creating canisters...
Creating canister internet_identity...
internet_identity canister created with canister id: avqkn-guaaa-aaaaa-qaaea-cai
Building canisters...
Error: Failed while trying to deploy canisters.
Caused by: Failed while trying to deploy canisters.
  Failed to build all canisters.
    Failed while trying to build all canisters.
      Failed to download https://github.com/dfinity/internet-identity/releases/latest/download/internet_identity_dev.wasm to /Users/moritz/projects/ii_integration/.dfx/local/canisters/internet_identity/download-internet_identity_dev.wasm.
        Failed to download from url: https://github.com/dfinity/internet-identity/releases/latest/download/internet_identity_dev.wasm.
          Not found: https://github.com/dfinity/internet-identity/releases/latest/download/internet_identity_dev.wasm

[frederikrothenberger]

Thanks, good catch!