Fix broken internal link

[bwignall]

Current version of the link 404s.

[satorg]

Thanks!

[satorg]

FutureSuite on JVM is failing again (link)

But despite of that, seems that the site generation step is not happy about this change (link):

[error] laika.api.errors.InvalidDocuments: One or more invalid documents:
[error] /CONTRIBUTING.md
[error] 
[error]   [136]: unresolved internal reference: docs/guidelines.md
[error] 
[error]   [See guidelines](docs/guidelines.md).
[error]   ^
[error] 	at laika.api.errors.InvalidDocuments$.$anonfun$from$7(errors.scala:125)
[error] 	at scala.Option.map(Option.scala:230)
[error] 	at laika.api.errors.InvalidDocuments$.from(errors.scala:125)
[error] 	at laika.api.errors.InvalidDocuments$.$anonfun$from$4(errors.scala:117)
[error] 	at scala.util.Either.fold(Either.scala:191)
[error] 	at laika.api.errors.InvalidDocuments$.from(errors.scala:117)
[error] 	at laika.io.internal.runtime.ParserRuntime$.rewriteTree$1(ParserRuntime.scala:135)
[error] 	at laika.io.internal.runtime.ParserRuntime$.$anonfun$run$35(ParserRuntime.scala:171)
[error] 	at flatMap @ laika.io.internal.runtime.ParserRuntime$.$anonfun$run$34(ParserRuntime.scala:170)
[error] 	at flatMap @ laika.io.internal.runtime.ParserRuntime$.$anonfun$run$33(ParserRuntime.scala:169)
[error] 	at mapN @ laika.helium.internal.builder.HeliumInputBuilder$.build(HeliumInputBuilder.scala:148)
[error] 	at unsafeRunSync @ laika.sbt.Settings$.$anonfun$parserConfig$1(Settings.scala:103)
[error] (docs / laikaSite) laika.api.errors.InvalidDocuments: One or more invalid documents:
[error] /CONTRIBUTING.md
[error] 
[error]   [136]: unresolved internal reference: docs/guidelines.md
[error] 
[error]   [See guidelines](docs/guidelines.md).
[error]   ^