How to Convert NPN to PNP transistor?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Modified schematic. LED is on when probe is dry.

Move the LED as shown in Figure 1.


When the probes are dry Q1 is off and the LED is lit via R2.
When the probes are wet Q1 is on and shorts out the LED.


The disadvantage with this simple fix is that current is wasted when the LED is off.

Your 100 &Omega; resistor value is a bit low (but is also the CircuitLab default). For an LED with a 2 V forward voltage you will drop 7 V across R2. A 1 k&Omega; resistor will limit the current to 7 mA which will be quite bright on a modern LED.