RFR: JDK-8272667: substandard error messages from the docs build

[Jonathan Gibbons]

Please review a small (delete one character) change to improve the error messages reported when bad HTML is detected when post-processing the output from pandoc to generate the docs.

The change is just to pass the filename as an argument to the command, instead of just piping the input to stdin. As a result, the name of any file containing bad input is reported in the message, instead of the message simply referring to `<stdin>`

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Commit messages:
 - JDK-8272667: substandard error messages from the docs build

Changes: https://git.openjdk.java.net/jdk/pull/5175/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5175&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272667
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5175.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5175/head:pull/5175

PR: https://git.openjdk.java.net/jdk/pull/5175