Fix minor typo in youtube tag

Author: mirekdlugosz

pelican/plugins/liquid_tags/youtube.py

@@ -119,7 +119,7 @@ def youtube(preprocessor, tag, markup):
                 </span>
             """.strip()
     else:
-        raise ValueError(f'Error processing input, " "expected syntax: {SYNTAX}')
+        raise ValueError(f"Error processing input, expected syntax: {SYNTAX}")
 
     return youtube_out