[doc] Bump fuzzer child process memory limit

Summary: The doc recommends 52MB but this is not sufficient.

Test Plan:
Setup the fuzzer according to the doc and then:
Fails with a memory limit error:

${AFLPATH}/afl-fuzz -i ${DIR_FUZZ_IN}/${FUZZ_TARGET} -o ${AFLOUT}/${FUZZ_TARGET} -m52 -- src/test/fuzz/${FUZZ_TARGET}

Succeeds:

${AFLPATH}/afl-fuzz -i ${DIR_FUZZ_IN}/${FUZZ_TARGET} -o ${AFLOUT}/${FUZZ_TARGET} -m80 -- src/test/fuzz/${FUZZ_TARGET}

I tested with FUZZ_TARGET=block_header and FUZZ_TARGET=cashaddr with the same results.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12693