Clarify docstring of extract command

pdfca.py

@@ -99,7 +99,7 @@ def cut(name, binary, format):
 def extract(directory, binary, format):
     """Scrape text from pages of files in "input" folder.
     Requires DIRECTORY (whether relative or absolute).
-    Use ./ as DIRECTORY to process files in the pdfca folder."""
+    Use "./" as DIRECTORY to process files in the pdfca folder."""
     binary = binary + format
     load_df(binary)
     os.chdir(directory)