docs: Fix BPF_HISTGRAM typo in reference guide
docs: Fix BPF_HISTGRAM typo in reference guide

Fix typo in reference guide, should be BPF_HISTOGRAM.
brendangregg/bcc,iovisor/bcc,brendangregg/bcc,iovisor/bcc,brendangregg/bcc,brendangregg/bcc,iovisor/bcc,iovisor/bcc,brendangregg/bcc,iovisor/bcc