Use this: Find
\r(?=\D)
Replace: a single space. If your References use a unique paragraph style, it's best to include that in the Find What formatting (this works the same as in regular Text find&change).
These codes are, uh, fairly advanced They mean:
\r - a regular Hard Return
(?= ...) - which must be followed by ... (what's between the parentheses) - a Positive Lookahead
\D - any character NOT a digit (the inverse of what is suggested above: \d is "only a digit")
So, in plain English, this will "find a return that is followed by a character which is not a digit". Precisely what you asked for!