docs: fix indexOf typo (#3526)

Author: UziTech

docs/USING_PRO.md

@@ -388,7 +388,7 @@ An **inline-level** extension will be handled inside each block-level token, bef
 <dt><code><strong>start</strong>(<i>string</i> src)</code></dt>
 <dd>A function that returns the index of the next potential start of the custom token.
 
-The index can be the result of a <code>src.match().index</code>, or even a simple <code>src.index()</code>. Marked will use this function to ensure that it does not skip over any text that should be part of the custom token.</dd>
+The index can be the result of a <code>src.match().index</code>, or even a simple <code>src.indexOf()</code>. Marked will use this function to ensure that it does not skip over any text that should be part of the custom token.</dd>
 
 <dt><code><strong>tokenizer</strong>(<i>string</i> src, <i>array</i> tokens)</code></dt>
 <dd>A function that reads string of Markdown text and returns a generated token. The token pattern should be found at the beginning of the <code>src</code> string. Accordingly, if using a Regular Expression to detect a token, it should be anchored to the string start (`^`). The <code>tokens</code> parameter contains the array of tokens that have been generated by the lexer up to that point, and can be used to access the previous token, for instance.