fix a typo s/overrided/overridden/ in help for terrains

data/core/help.cfg

@@ -496,7 +496,7 @@ Terrains come in two types: <italic>text='basic'</italic> terrain types and <ita
 
 You can see what basic types are associated to a mixed type by mousing over its hex and checking the terrain type icons displayed in the upper right, under the default theme." + _"
 
-Some units have <italic>text='defense caps'</italic> for a particular basic terrain type. In that case, the 'best defense' behavior is overrided -- they can never recieve a higher defense rating than the lowest defense cap value for any mixed terrain type which includes that basic type. For example, the loyalist cavalryman has a defense rating of 30% on forests, and a defense cap for forests. Thus, on forested hills, he has a defense rating of 30% rather than 40%, because the mixed rating cannot exceed the cap.
+Some units have <italic>text='defense caps'</italic> for a particular basic terrain type. In that case, the 'best defense' behavior is overridden -- they can never recieve a higher defense rating than the lowest defense cap value for any mixed terrain type which includes that basic type. For example, the loyalist cavalryman has a defense rating of 30% on forests, and a defense cap for forests. Thus, on forested hills, he has a defense rating of 30% rather than 40%, because the mixed rating cannot exceed the cap.
 
 <header>text='Basic Terrain Types'</header>