Fix some spelling errors (mostly in comments).

src/api/dotnet/ArithExpr.cs

@@ -45,7 +45,7 @@ namespace Microsoft.Z3
 
         private static ArithExpr MkNum(ArithExpr e, double d) { return (ArithExpr)e.Context.MkNumeral(d.ToString(), e.Context.MkRealSort()); }        
 
-        /// <summary> Operator overloading for arithmetical divsion operator (over reals) </summary>
+        /// <summary> Operator overloading for arithmetical division operator (over reals) </summary>
         public static ArithExpr operator /(ArithExpr a, ArithExpr b) { return a.Context.MkDiv(a, b); }
 
         /// <summary> Operator overloading for arithmetical operator </summary>

src/api/dotnet/Context.cs

@@ -2459,7 +2459,7 @@ namespace Microsoft.Z3
 
         #endregion
 
-        #region Sequence, string and regular expresions
+        #region Sequence, string and regular expressions
 
         /// <summary>
         /// Create the empty sequence.
@@ -3131,7 +3131,7 @@ namespace Microsoft.Z3
         /// <summary>
         /// Create a bit-vector numeral.
         /// </summary>
-        /// <param name="bits">An array of bits representing the bit-vector. Least signficant bit is at position 0.</param>
+        /// <param name="bits">An array of bits representing the bit-vector. Least significant bit is at position 0.</param>
         public BitVecNum MkBV(bool[] bits)
         {
             Contract.Ensures(Contract.Result<BitVecNum>() != null);

src/api/dotnet/Expr.cs

@@ -842,7 +842,7 @@ namespace Microsoft.Z3
         public string String { get { return Native.Z3_get_string(Context.nCtx, NativeObject); } }
 
         /// <summary>
-        /// Check whether expression is a concatentation.
+        /// Check whether expression is a concatenation.
         /// </summary>
         /// <returns>a Boolean</returns>
         public bool IsConcat { get { return IsApp && FuncDecl.DeclKind == Z3_decl_kind.Z3_OP_SEQ_CONCAT; } }