1. Open the file
  2. Click CTRL + F
  3. Select "Current document" in "Find in" (You can also select the folder if you have multiple files)
  4. Search in "Source code"
  5. Tick "Use regular expression"
  6. Type "[ ]{2,}" (without quotes) in "Find"
  7. Type " " (without quotes) in "Replace"
  8. Press "Replace All"

You can even make this a site-wide thing in dreamweaver by searching 'All Local Files and Folders' instead of just the open file in step 4.