Fix typos in comments (#402)

Author: MaherJendoubi

eng/common/tools.ps1

@@ -84,7 +84,7 @@ function Exec-Process([string]$command, [string]$commandArgs) {
     return $global:LASTEXITCODE = $process.ExitCode
   }
   finally {
-    # If we didn't finish then an error occured or the user hit ctrl-c.  Either
+    # If we didn't finish then an error occurred or the user hit ctrl-c.  Either
     # way kill the process
     if (-not $finished) {
       $process.Kill()

src/csharp/Microsoft.Spark/Sql/Column.cs

@@ -490,7 +490,7 @@ public Column SubStr(int startPos, int len)
         /// Contains the other element. Returns a boolean column based on a string match.
         /// </summary>
         /// <param name="other">
-        /// The object that is used to check for existance in the current column.
+        /// The object that is used to check for existence in the current column.
         /// </param>
         /// <returns>New column after checking if the column contains object other</returns>
         public Column Contains(object other)