docs: fix wrong corpus dir in README

[Marcono1234]

Was originally mentioned in #965 (comment), but was apparently overlooked by accident.

For the corpus directory <package> and <FuzzTestClass> are combined (with a .) to a single name; they not separate directories.

[simonresch]

Good spot! Thanks for looking this over and providing feedback. Very much appreciated. :)

@oetr We seem to have some flakyness in the new ArgumentsMutatorFuzzTest. It failed here with an out of memory finding:

ChangeBit-Custom-EraseBytes-Custom-ShuffleBytes-Custom-
#9249	NEW    cov: 1178 ft: 4214 corp: 459/73Kb lim: 3784 exec/s: 924 rss: 839Mb L: 113/3773 MS: 6 InsertByte-Custom-EraseBytes-Custom-InsertRepeatedBytes-Custom-
DEDUP_TOKEN: 7fb7f6a3d702697c

== Java Exception: com.code_intelligence.jazzer.api.FuzzerSecurityIssueLow: Out of memory (use '-Xmx1612m' to reproduce)
Caused by: java.lang.OutOfMemoryError: Java heap space
	at com.google.protobuf.CodedInputStream$StreamDecoder.readRawBytesSlowPathOneChunk(CodedInputStream.java:2945)
	at com.google.protobuf.CodedInputStream$StreamDecoder.readBytesSlowPath(CodedInputStream.java:3009)
	at com.google.protobuf.CodedInputStream$StreamDecoder.readBytes(CodedInputStream.java:2420)
	at com.google.protobuf.UnknownFieldSet$Builder.mergeFieldFrom(UnknownFieldSet.java:513)
	at com.google.protobuf.UnknownFieldSet$Builder.mergeFrom(UnknownFieldSet.java:490)
	at com.google.protobuf.UnknownFieldSet$Builder.mergeFrom(UnknownFieldSet.java:603)
	at com.google.protobuf.UnknownFieldSet$Builder.mergeFrom(UnknownFieldSet.java:303)
	at com.google.protobuf.CodedInputStream$StreamDecoder.readGroup(CodedInputStream.java:2347)
	at com.google.protobuf.UnknownFieldSet$Builder.mergeFieldFrom(UnknownFieldSet.java:517)
	at com.google.protobuf.GeneratedMessageV3$Builder.parseUnknownField(GeneratedMessageV3.java:871)
	at com.google.protobuf.BoolValue$Builder.mergeFrom(BoolValue.java:330)
	at com.google.protobuf.BoolValue$Builder.mergeFrom(BoolValue.java:213)

I'll retry the job for now.