Fix typo: c_float represents to C float type.

Doc/lib/libctypes.tex

@@ -2177,7 +2177,7 @@ optional float initializer.
 \end{classdesc*}
 
 \begin{classdesc*}{c_float}
-Represents the C double datatype. The constructor accepts an
+Represents the C float datatype. The constructor accepts an
 optional float initializer.
 \end{classdesc*}