Search for this:
([a-z])</p>\s+<p class="whatever">([a-z])
The \s+ in the middle is for the space between the two lines and is necessary to find what you're looking for.
Replace with this:
\1 \2
You might also want to do two searches and drop the ([a-z]) at the end, though, because you'll find there are a lot of lines that need to be connected where the second line doesn't start with a small letter.