share: remove unexpected heading from "verbose" container in help test
`test-gendoc-*.t` have been failing for me since 91425656e2b1 (share:
add documentation about share-safe mode in `hg help -e share`,
2020-11-27) with this kind of output:
```
--- /usr/local/google/home/martinvonz/hg/tests/test-gendoc-ru.t
+++ /usr/local/google/home/martinvonz/hg/tests/test-gendoc-ru.t.err
@@ -2,3 +2,9 @@
$ $TESTDIR/check-gendoc ru
checking for parse errors
+ gendoc.txt:12818: (SEVERE/4) Unexpected section title.
+
+ Sharing requirements and configs of source repository with shares
+ -----------------------------------------------------------------
+ Exiting due to level-4 (SEVERE) system message.
+ [1]
```
This patch fixes that.
Differential Revision: https://phab.mercurial-scm.org/D9552