fix: remove double slash in include path for ninf.h in betaln

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---

Author: Planeshifter

lib/node_modules/@stdlib/math/base/special/betaln/src/main.c

@@ -33,7 +33,7 @@
 #include "stdlib/math/base/special/ln.h"
 #include "stdlib/constants/float64/ln_sqrt_two_pi.h"
 #include "stdlib/constants/float64/pinf.h"
-#include "stdlib//constants/float64/ninf.h"
+#include "stdlib/constants/float64/ninf.h"
 #include <stdint.h>
 
 static const double ALGMCS[ 15 ] = {