[stdlib] Fix thinko

(cherry picked from commit 47109ac)

Author: lorentey

stdlib/public/core/StringGutsRangeReplaceable.swift

@@ -466,7 +466,7 @@ extension _StringGuts {
     _internalInvariant(
       subrange.lowerBound >= startIndex && subrange.upperBound <= endIndex)
 
-    guard _slowPath(isUTF8) else {
+    guard _fastPath(isUTF8) else {
       // UTF-16 string. The mutation will convert this to the native UTF-8
       // encoding, so we need to do some extra work to preserve our bounds.
       let utf8StartOffset = String(self).utf8.distance(