[nim/en] Clarify Nim zero value for uninitialized variables (#4573)

nim.html.markdown

@@ -188,7 +188,7 @@ echo "I'm thinking of a number between 41 and 43. Guess which!"
 let number: int = 42
 var
   raw_guess: string
-  guess: int
+  guess: int # Variables in Nim are always initialized with a zero value
 while guess != number:
   raw_guess = readLine(stdin)
   if raw_guess == "": continue # Skip this iteration