Fixed typo in the method summary (#296)

Author: pkulikov

src/Microsoft.ML/Models/RegressionMetrics.cs

@@ -71,7 +71,7 @@ internal static List<RegressionMetrics> FromOverallMetrics(IHostEnvironment env,
  public double L2 { get; private set; }
 
  /// <summary>
- /// Gets the root mean square loss (or RMC) which is the square root of the L2 loss.
+ /// Gets the root mean square loss (or RMS) which is the square root of the L2 loss.
  /// </summary>
  public double Rms { get; private set; }