Fix a typo found by Eric D. Moyer, closes SF 807813.

Will backport to 2.3.

Doc/dist/dist.tex

@@ -1477,7 +1477,7 @@ get_special_folder_path(csidl_string)
 
 This function can be used to retrieve special folder locations on
 Windows like the Start Menu or the Desktop.  It returns the full path
-to the folder.  'csidl_string' must be on of the following strings:
+to the folder.  'csidl_string' must be one of the following strings:
 
 \begin{verbatim}
 "CSIDL_APPDATA"