Markup: add alt text to Ecma logo (#3152)

This is the only `<img>` element without an alt text in the entire
specification, so add an alt text.

spec.html

@@ -58,7 +58,7 @@
   location: https://tc39.es/ecma262/
   markEffects: true
 </pre>
-<p><img src="img/ecma-logo.svg" id="ecma-logo"></p>
+<p><img src="img/ecma-logo.svg" id="ecma-logo" alt="Ecma International logo"></p>
 <div id="metadata-block">
   <h1>About this Specification</h1>
   <p>The document at <a href="https://tc39.es/ecma262/">https://tc39.es/ecma262/</a> is the most accurate and up-to-date ECMAScript specification. It contains the content of the most recent yearly snapshot plus any <a href="https://github.com/tc39/proposals/blob/HEAD/finished-proposals.md">finished proposals</a> (those that have reached Stage&nbsp;4 in the <a href="https://tc39.es/process-document/">proposal process</a> and thus are implemented in several implementations and will be in the next practical revision) since that snapshot was taken.</p>