[CMAKE] Fix version in the man2html template

Summary:
The template uses PROJECT_VERSION, which might be empty for subprojects.
What we want here is the top level project version, so the right
variable is CMAKE_PROJECT_VERSION.

Test Plan:

cmake -GNinja ..

Check the script has the correct version number.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7771