system/mandoc: off-by-one

Found on an installed x86_64 KDE environment from snapshot 20231027:

added BETA5 label

Something to do with makewhatis?

Here's running ours, then the built-from-source-no-patches version:

Problem triggered by:

makewhatis -a

problem avoided by:

makewhatis

See also:

• https://gitlab.alpinelinux.org/alpine/aports/-/issues/12317

closed with commit 53dfec0e

mentioned in commit 53dfec0e