[gardening] fix copy-pasta

Author: glessard

stdlib/public/core/Slice.swift

@@ -309,7 +309,7 @@ extension Slice: MutableCollection where Base: MutableCollection {
         _precondition(
           slice.baseAddress == copy.baseAddress &&
           slice.count == copy.count,
-          "Slice.withUnsafeMutableBufferPointer: replacing the buffer is not allowed")
+          "Slice.withContiguousMutableStorageIfAvailable: replacing the buffer is not allowed")
       }
       return try body(&slice)
     }

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -472,7 +472,7 @@ extension Unsafe${Mutable}BufferPointer {
     let (oldBase, oldCount) = (self.baseAddress, self.count)
     defer { 
       _debugPrecondition((oldBase, oldCount) == (self.baseAddress, self.count),
-      "UnsafeMutableBufferPointer.withUnsafeMutableBufferPointer: replacing the buffer is not allowed")
+      "UnsafeMutableBufferPointer.withContiguousMutableStorageIfAvailable: replacing the buffer is not allowed")
     } 
     return try body(&self)
   }