fix missing space before inline math

[mdbmes]

This fixes a Markdown rendering error in the client-side-encryption spec due to a typo. The problem was much more noticeable on GitHub than on mkdocs (readthedocs):

• on GitHub, the word was no longer interpreted as math, and it rendered in plain text as "joining$dbName$"
• on mkdocs the math markup rendered normally still, so it appeared as "joiningdbName" with "dbName" in math italics, making the missing space somewhat hard to see

[blink1073]

Yay, readthedocs preview in PRs is back!