Update used-in-an-unusual-or-restricted-way.html (#3707)

closes #3487 by making the reference WCAG 2 and not WCAG 2.1.
Similar to effort in #3585
It is unclear to me if the change needs to be made in the hmtl or in one
of these source files, where the definition also occurs, so citing them
as part of the change /wcag20/sources/wcag2-src.xml
/wcag20/sources/wcag2-src.REC.xml

Note that this change is based on existing non-normative information
under [Background on WCAG
2](https://www.w3.org/TR/WCAG22/#background-on-wcag-2) in the existing
TR, which reads in part:
>Where this document refers to WCAG 2 it is intended to mean any and all
versions of WCAG that start with 2.

(cherry picked from commit eabc5e3)

Author: mbgower

guidelines/terms/20/used-in-an-unusual-or-restricted-way.html

@@ -1,14 +1,16 @@
 <dt><dfn id="dfn-used-in-an-unusual-or-restricted-way">used in an unusual or restricted way</dfn></dt>
 <dd>
-   
+
    <p>words used in such a way that requires users to know exactly which definition to apply
       in order to understand the content correctly
    </p>
-   
-   <aside class="example"><p>The term "gig" means something different if it occurs in a discussion of music concerts
-      than it does in article about computer hard drive space, but the appropriate definition
-      can be determined from context. By contrast, the word "text" is used in a very specific
-      way in WCAG 2.1, so a definition is supplied in the glossary.
-   </p></aside>
-   
+
+   <aside class="example">
+      <p>The term "gig" means something different if it occurs in a discussion of music concerts
+         than it does in article about computer hard drive space, but the appropriate definition
+         can be determined from context. By contrast, the word "text" is used in a very specific
+         way in WCAG 2, so a definition is supplied in the glossary.
+      </p>
+   </aside>
+
 </dd>