[cleanup] fix small mistake in doc

[hogoww]

Hello,

I'm reading a bunch of documentation and found a few mistakes.
The committed one was a fairly easy fix, but others are not.
I'd love to do more, but I'll need some support for that !

Here are the other ones I found so far:
https://github.com/graalvm/native-build-tools/blob/master/docs/src/docs/asciidoc/maven-plugin.adoc

• There's another gradle reference at the start of the file :highlighjsdir: {gradle-relative-srcdir}/highlight that doesn't seem to render. I don't know this syntax.
• The javadoc link at the end of the file is broken.

The snippet in those two files seems reversed.

• this one references gradle instead of maven: https://github.com/graalvm/native-build-tools/blob/master/native-maven-plugin/README.md
• this one references gradle instead of maven: https://github.com/graalvm/native-build-tools/blob/master/native-maven-plugin/README.md
  If someone could confirm that this is indeed just reversed, I can commit it quickly.

I have signed the OCA a couple of years ago, seems to never have been fvalidated.
I did it again yesterday.

Have a nice day !

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