CONTRIBUTING_zh.md: all URLs use full-width colons (：), breaking links and copy-paste commands

[simpleqt]

Every URL in CONTRIBUTING_zh.md (12 lines, 13 occurrences) uses the full-width colon '：' (U+FF1A) instead of ':', e.g. line 11: https：//github.com/orgs/agentscope-ai/projects/2. GitHub does not autolink these, and the git clone / remote-add commands at lines 58/60 fail verbatim when copied. The English CONTRIBUTING.md uses ASCII colons throughout.

Proposed fix: replace https： with https: on all 12 lines. Happy to send a PR.