Fix a variable name in winreg docs.

 (backport from rev. 56091)

Doc/lib/libwinreg.tex

@@ -321,7 +321,7 @@ This module offers the following functions:
  \var{key} is an already open key, or one of the predefined 
  \constant{HKEY_*} constants.
 
- \var{sub_key} is a string that names the subkey with which the 
+ \var{value_name} is a string that names the subkey with which the 
  value is associated.
 
  \var{type} is an integer that specifies the type of the data.