[Graal] Update java-allocation-instrumenter.jar handling in graalunit README.md

The download instruction in README.md says to download java-allocation-instrumenter-8f0db117e64e.jar:
http://hg.openjdk.java.net/jdk/jdk/file/00c0906bf4d1/test/hotspot/jtreg/compiler/graalunit/README.md#l24

However, that produces build failure:

$ CONF=linux-x86_64-server-fastdebug make run-test TEST=compiler/graalunit
Building target 'run-test' in configuration 'linux-x86_64-server-fastdebug'
Creating interim jimage
Compiling 1177 files for BUILD_VM_COMPILER_TESTS
Creating support/test/graalunit/com.oracle.mxtool.junit.jar
/home/shade/trunks/jdk-jdk/src/jdk.internal.vm.compiler/share/classes/org.graalvm.compiler.core.test/src/org/graalvm/compiler/core/test/AllocSpy.java:40: error: package com.google.monitoring.runtime.instrumentation does not exist
import com.google.monitoring.runtime.instrumentation.AllocationRecorder;
                                                    ^
/home/shade/trunks/jdk-jdk/src/jdk.internal.vm.compiler/share/classes/org.graalvm.compiler.core.test/src/org/graalvm/compiler/core/test/AllocSpy.java:41: error: package com.google.monitoring.runtime.instrumentation does not exist
import com.google.monitoring.runtime.instrumentation.Sampler;
                                                    ^
/home/shade/trunks/jdk-jdk/src/jdk.internal.vm.compiler/share/classes/org.graalvm.compiler.core.test/src/org/graalvm/compiler/core/test/AllocSpy.java:237: error: cannot find symbol
    static class GraalContextSampler implements Sampler {
                                                ^
  symbol: class Sampler
  location: class AllocSpy
3 errors

It seems to pass if you drop the hex suffix from the downloaded JAR:

$ mv java-allocation-instrumenter-8f0db117e64e.jar java-allocation-instrumenter.jar