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.

(cherry picked from commit 9249aca)

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>