Fix `toolchain` typo in bazel-and-java.md
Fix `toolchain` typo in bazel-and-java.md

The "Java and Bazel" page twice referenced a method
`register_toolchain` which contains a typo. The method's correct name is
`register_toolchains`, with a plural "s".

Closes #15953.

PiperOrigin-RevId: 463243967
Change-Id: I150d4b83110dafd7df2ec881b5dca3b540749c49

cushon/bazel,bazelbuild/bazel,bazelbuild/bazel,cushon/bazel,bazelbuild/bazel,katre/bazel,katre/bazel,bazelbuild/bazel,bazelbuild/bazel,bazelbuild/bazel,cushon/bazel,cushon/bazel,katre/bazel,katre/bazel,cushon/bazel,katre/bazel,cushon/bazel,katre/bazel