help: Update keyboard page to 3.38

Add subscript link to the keyboard page.

Ref: #185

help/C/keyboard.page

@@ -4,6 +4,12 @@
 
     <info>
         <link type="guide" xref="index#ui"/>
+        <revision pkgversion="3.38.0" date="2020-09-25" status="review"/>
+        <credit type="editor">
+          <name>Pranali Deshmukh</name>
+          <email>pranali21293@gmail.com</email>
+          <years>2020</years>
+        </credit>
     </info>
 
 	<title>Using the Keyboard</title>
@@ -37,6 +43,6 @@
     </tr>
     </table>
     <p>
-    To enter <link xref="superscript">superscript numbers</link> use <keyseq><key>Ctrl</key>number</keyseq>, for subscript use <keyseq><key>Alt</key>number</keyseq>.
+    To enter <link xref="superscript">superscript numbers</link> use <keyseq><key>Ctrl</key>number</keyseq>, for <link xref="superscript">subscript numbers</link> use <keyseq><key>Alt</key>number</keyseq>.
     </p>
 </page>