Remove white space between list in v0.21.5 release notes

Summary: TIL white space between itmes breaks lists in markdown.

Test Plan:

Read it.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6078