dev-bazel: Fix headlines for java 9 and java 10 support sections

Instead of headlines, anchors were created out of context.

Convert the anchors to headlines, within a "Java" subsection of
the "Prerequisities" section.

Change-Id: Ifa017ecbd2dd111166cfc488f3b5a5613d7fa14d
diff --git a/Documentation/dev-bazel.txt b/Documentation/dev-bazel.txt
index 0ecc820..4978a55 100644
--- a/Documentation/dev-bazel.txt
+++ b/Documentation/dev-bazel.txt
@@ -14,7 +14,12 @@
 * zip, unzip
 * gcc
 
-[[Java 10 support]]
+[[java]]
+=== Java
+
+[[java-10]]
+==== Java 10 support
+
 Java 10 is supported through vanilla java toolchain
 link:https://docs.bazel.build/versions/master/toolchains.html[Bazel option].
 To build Gerrit with Java 10, specify vanilla java toolchain and provide
@@ -38,7 +43,9 @@
   javaOptions = --add-opens=jdk.management/com.sun.management.internal=ALL-UNNAMED
 ```
 
-[[Java 9 support]]
+[[java-9]]
+==== Java 9 support
+
 Java 9 is supported through alternative java toolchain
 link:https://docs.bazel.build/versions/master/toolchains.html[Bazel option].
 The Java 9 support is backwards compatible. Java 8 is still the default.