Fix a typo

Doc/lib/libos.tex

@@ -918,7 +918,7 @@ fractions of a second in a time stamp can use this function to have
 time stamps represented as floats. Whether they will actually observe
 non-zero fractions depends on the system.
 
-Future Python releases will change the default of this settings;
+Future Python releases will change the default of this setting;
 applications that cannot deal with floating point time stamps can then
 use this function to turn the feature off.