[Documentation] Fix UnsafeMutablePointer's documentation

(cherry picked from commit f96ae2f)

Author: Kyle-Ye

stdlib/public/core/UnsafePointer.swift

@@ -611,7 +611,7 @@ public struct UnsafeMutablePointer<Pointee>: _Pointer {
     self.init(mutating: unwrapped)
   }
 
-  /// Creates an immutable typed pointer referencing the same memory as the
+  /// Creates a mutable typed pointer referencing the same memory as the
   /// given mutable pointer.
   ///
   /// - Parameter other: The pointer to convert.
@@ -620,7 +620,7 @@ public struct UnsafeMutablePointer<Pointee>: _Pointer {
    self._rawValue = other._rawValue
   }
 
-  /// Creates an immutable typed pointer referencing the same memory as the
+  /// Creates a mutable typed pointer referencing the same memory as the
   /// given mutable pointer.
   ///
   /// - Parameter other: The pointer to convert. If `other` is `nil`, the