Fix a typo

Doc/lib/libcfgparser.tex

@@ -50,7 +50,7 @@ others.
 
 Sections are normally stored in a builtin dictionary. An alternative
 dictionary type can be passed to the \class{ConfigParser} constructor.
-For example, if a dictionary type is passed that sorts is keys,
+For example, if a dictionary type is passed that sorts its keys,
 the sections will be sorted on write-back, as will be the keys within
 each section.