[guide] fix grammar

Author: morninj

README.md

@@ -636,7 +636,7 @@ Other Style Guides
     ```
 
   <a name="strings--eval"></a><a name="6.5"></a>
-  - [6.4](#strings--eval) Never use `eval()` on a string, it opens too many vulnerabilities. eslint: [`no-eval`](https://eslint.org/docs/rules/no-eval)
+  - [6.4](#strings--eval) Never use `eval()` on a string; it opens too many vulnerabilities. eslint: [`no-eval`](https://eslint.org/docs/rules/no-eval)
 
   <a name="strings--escaping"></a>
   - [6.5](#strings--escaping) Do not unnecessarily escape characters in strings. eslint: [`no-useless-escape`](https://eslint.org/docs/rules/no-useless-escape)