Fix build output directory for ref docs in CONTRIBUTING.md

[cprayer]

spring-framework/gradle/docs.gradle

Lines 150 to 168 in fd49c8f

	/**
	* Generate the Spring Framework Reference documentation from "src/docs/asciidoc"
	* in "build/docs/ref-docs/html5".
	*/
	asciidoctor {
	baseDirFollowsSourceDir()
	configurations 'asciidoctorExt'
	sources {
	include '*.adoc'
	}
	outputDir "$buildDir/docs/ref-docs/html5"
	logDocuments = true
	resources {
	from(sourceDir) {
	include 'images/*.png', 'css/**', 'js/**'
	}
	from extractDocResources
	}
	}

(fd49c8f is latest commit of master branch)
As you can see, I think it should be build/docs/ref-docs/html5 directory, not a build/asciidoc/html5 directory.

[sbrannen]

Good catch.

This has been merged into master.

Thanks