Add missing space to checksum string in sample code (#441)

1. Added the missing space to the checksum string.
2. Add `w` flag to `sha256sum` so that user is warned if a formatting issue (like the missing space) occurs again in the future.

Author: gldraphael

2.1/README.md

@@ -26,7 +26,7 @@ Sample code to install NodeJS on your own:
 ENV NODE_VERSION 8.9.4
 ENV NODE_DOWNLOAD_SHA 21fb4690e349f82d708ae766def01d7fec1b085ce1f5ab30d9bda8ee126ca8fc
 RUN curl -SL "https://nodejs.org/dist/v${NODE_VERSION}/node-v${NODE_VERSION}-linux-x64.tar.gz" --output nodejs.tar.gz \
-    && echo "$NODE_DOWNLOAD_SHA nodejs.tar.gz" | sha256sum -c - \
+    && echo "$NODE_DOWNLOAD_SHA  nodejs.tar.gz" | sha256sum -cw - \
     && tar -xzf "nodejs.tar.gz" -C /usr/local --strip-components=1 \
     && rm nodejs.tar.gz \
     && ln -s /usr/local/bin/node /usr/local/bin/nodejs