Fix typo in Mode.toString (scala#26223)

## How much have you relied on LLM-based tools in this contribution?

LLM identifies this mistake. Full report:
https://folium.capless.cc/claude-review.html

## How was the solution tested?

Existing tests.
[Cherry-picked e018144]

Author: Linyxus

compiler/src/dotty/tools/dotc/core/Mode.scala

@@ -19,7 +19,7 @@ case class Mode(val bits: Int) extends AnyVal {
   def isExpr: Boolean = (this & PatternOrTypeBits) == None
 
   override def toString: String =
-    (0 until 31).filter(i => (bits & (1 << i)) != 0).map(modeName).mkString("Mode(", ",", ")")
+    (0 to 31).filter(i => (bits & (1 << i)) != 0).map(modeName).mkString("Mode(", ",", ")")
 
   def ==(that: Mode): Boolean = this.bits == that.bits
   def !=(that: Mode): Boolean = this.bits != that.bits