docs: add missing include

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Author: kgryte

lib/node_modules/@stdlib/math/base/assert/is-probability/README.md

@@ -102,6 +102,8 @@ for ( i = 0; i < x.length; i++ ) {
 Tests if a double-precision floating-point number is a probability.
 
 ```c
+#include <stdbool.h>
+
 bool out = stdlib_base_is_probability( 0.5 );
 // returns true