Fix typo in section 4.2.2 (#445)

Fixes #428

Author: gkatsev

index.bs

@@ -1713,7 +1713,7 @@ the next by a <a>WebVTT line terminator</a>.</p>
 
 <p>All <a>WebVTT caption or subtitle cue components</a> bar the HTML character reference may have
 one or more <dfn>cue component class names</dfn> attached to it by separating the cue component
-class name from the cue component start tag using the perido ('.') notation. The class name must
+class name from the cue component start tag using the period ('.') notation. The class name must
 immediately follow the "period" (.).</p>
 
 <p><dfn>WebVTT cue internal text</dfn> consists of an optional <a>WebVTT line terminator</a>,

index.html

@@ -2743,7 +2743,7 @@ <h4 class="heading settled" data-level="4.2.2" id="caption-text"><span class="se
    </ul>
    <p>All <a data-link-type="dfn" href="#webvtt-caption-or-subtitle-cue-components" id="ref-for-webvtt-caption-or-subtitle-cue-components-2">WebVTT caption or subtitle cue components</a> bar the HTML character reference may have
 one or more <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport="" id="cue-component-class-names">cue component class names</dfn> attached to it by separating the cue component
-class name from the cue component start tag using the perido ('.') notation. The class name must
+class name from the cue component start tag using the period ('.') notation. The class name must
 immediately follow the "period" (.).</p>
    <p><dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport="" id="webvtt-cue-internal-text">WebVTT cue internal text</dfn> consists of an optional <a data-link-type="dfn" href="#webvtt-line-terminator" id="ref-for-webvtt-line-terminator-21">WebVTT line terminator</a>,
 followed by zero or more <a data-link-type="dfn" href="#webvtt-caption-or-subtitle-cue-components" id="ref-for-webvtt-caption-or-subtitle-cue-components-3">WebVTT caption or subtitle cue components</a>, in any order, each