Editorial: Add missing period to initial value paragraphs (#386)

Author: arai-a

spec/immutable-data-structures.html

@@ -178,7 +178,7 @@ <h1>Tuple.of ( ..._items_ )</h1>
       </emu-clause>
       <emu-clause id="sec-tuple.prototype">
         <h1>Tuple.prototype</h1>
-        <p>The initial value of *Tuple.prototype* is %Tuple.prototype%</p>
+        <p>The initial value of *Tuple.prototype* is %Tuple.prototype%.</p>
         <p>This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.</p>
       </emu-clause>
     </emu-clause>
@@ -214,7 +214,7 @@ <h1>
 
       <emu-clause id="sec-tuple.prototype.constructor">
         <h1>Tuple.prototype.constructor</h1>
-        <p>The initial value of *Tuple.prototype.constructor* is the intrinsic object %Tuple%</p>
+        <p>The initial value of *Tuple.prototype.constructor* is the intrinsic object %Tuple%.</p>
       </emu-clause>
       <emu-clause id="sec-tuple.prototype.at">
         <h1>Tuple.prototype.at ( _index_ )</h1>