test/periph_hwrng: fixed output formatting

tests/periph_hwrng/main.c

@@ -31,7 +31,7 @@ int main(void)
     uint8_t buf[LIMIT];
 
     puts("\nHWRNG peripheral driver test\n");
-    printf("This test will print from 1 to %i random bytes about every"
+    printf("This test will print from 1 to %i random bytes about every "
            "second\n\n", LIMIT);
 
     puts("Initializing the HWRNG driver.\n");