Remove periods from end of definitions (#4359)

Partially addresses comments in #4122 by removing the periods from the
ends of the definitions for perimeter and single pointer.
This brings these definitions into the glossary style, where the
definition does not have terminal punctuation.

Author: mbgower

guidelines/terms/21/single-pointer.html

@@ -1,6 +1,6 @@
 <dt><dfn id="dfn-single-pointer">single pointer</dfn></dt>
 <dd>
-   <p>an input modality that only targets a single point on the page/screen at a time – such as a mouse, single finger on a touch screen, or stylus.</p>
+   <p>an input modality that only targets a single point on the page/screen at a time – such as a mouse, single finger on a touch screen, or stylus</p>
    <p class="note">Single pointer interactions include clicks, double clicks, taps, dragging motions, and single-finger swipe gestures. In contrast, multipoint interactions involve the use of two or more pointers at the same time, such as two-finger interactions on a touchscreen, or the simultaneous use of a mouse and stylus.</p>
 </dd>
 

guidelines/terms/22/perimeter.html

@@ -1,7 +1,7 @@
 <dt class="new"><dfn id="dfn-perimeter">perimeter</dfn></dt>
 <dd class="new">
 	<p class="change">New</p>	
-   <p>continuous line forming the boundary of a shape not including shared pixels, or the <a>minimum bounding box</a>, whichever is shortest.</p>
+   <p>continuous line forming the boundary of a shape not including shared pixels, or the <a>minimum bounding box</a>, whichever is shortest</p>
   <aside class="example"><p>The perimeter calculation for a 2 CSS pixel perimeter around a rectangle is 4<em>h</em>+4<em>w</em>, where <em>h</em> is the height and <em>w</em> is the width. For a 2 CSS pixel perimeter around a circle it is 4𝜋<em>r</em>.</p></aside>
 </dd>