scripts/managen: fix parsing of markdown code sections

scripts/managen: fix parsing of markdown code sections

- Terminate a code section before parsing a heading line.

Prior to this change when a code line (eg "    code") was followed
by a heading line (eg "## heading") the code section in the output
was terminated after converting the header instead of before. That led
to some weird formatting outputs depending on the nroff or roffit etc.

With this change:

.nf
curl \--expand\-url https.//example.com/{{url:trim}}
.fi
.IP json

Without this change:

.nf
curl \--expand\-url https.//example.com/{{url:trim}}
.IP json
.fi

Closes https://github.com/curl/curl/pull/16345