doc/protocols/file: fix semantical reverse

Change "read" to "write" in introductory description.
Spotted-by: Alexander Strasser <eclipse7@gmx.net>

doc/protocols.texi

@@ -117,7 +117,7 @@ ffmpeg -i "data:image/gif;base64,R0lGODdhCAAIAMIEAAAAAAAA//8AAP//AP/////////////
 
 File access protocol.
 
-Allow to read from or read to a file.
+Allow to read from or write to a file.
 
 A file URL can have the form:
 @example