[docs] Update documentation for bugprone-misplaced-widening-cast

[vabridgers]

Summary

The default setting for CheckImplicitCasts was changed in
https://reviews.llvm.org/D32164 but the documentation was not updated.
This simple change just syncs the documentation with the behavior of
that checker.

[aaron.ballman]

LGTM, thank you!

clang-tools-extra/docs/clang-tidy/checks/bugprone-misplaced-widening-cast.rst

double f(float x) {
         return (double)(x * 10.0f);
     }
 
 Options
 -------
 
 .. option:: CheckImplicitCasts
 
-   If `true`, enables detection of implicit casts. Default is `true`.
+   If `true`, enables detection of implicit casts. Default is `false`.