os: format readme, fix markdown inside html (#21286)

vlib/os/README.md

@@ -4,22 +4,20 @@
 command line arguments, reading/writing files, listing folders,
 handling processes etc.
 
-* * *
-
+---
 
 ### Security advice related to TOCTOU attacks
 
-A few `os` module functions can lead to the <b>TOCTOU</b> vulnerability if used incorrectly. 
-<b>TOCTOU</b> (Time-of-Check-to-Time-of-Use problem) can occur when a file, folder or similar 
+A few `os` module functions can lead to the **TOCTOU** vulnerability if used incorrectly.
+**TOCTOU** (Time-of-Check-to-Time-of-Use problem) can occur when a file, folder or similar
 is checked for certain specifications (e.g. read, write permissions) and a change is made
 afterwards.
 In the time between the initial check and the edit, an attacker can then cause damage.
 The following example shows an attack strategy on the left and an improved variant on the right
-so that <b>TOCTOU</b> is no longer possible.
+so that **TOCTOU** is no longer possible.
 
-
-<b>Example</b>
-<i>Hint</i>: `os.create()` opens a file in write-only mode
+**Example** <br>
+*Hint*: `os.create()` opens a file in write-only mode
 
 <table>
 <tr>
@@ -39,6 +37,7 @@ if os.is_writable("file") {
     f.close()
 }
 ```
+
 </td>
 <td>
 
@@ -52,14 +51,15 @@ mut f := os.create('path/to/file') or {
 
 f.close()
 ```
+
 </td>
 </tr>
 </table>
 
-<b> Proven affected functions </b></br>
+**Proven affected functions** <br>
 The following functions should be used with care and only when used correctly.
 
-* os.is_readable()
-* os.is_writable()
-* os.is_executable()
-* os.is_link()
+- os.is_readable()
+- os.is_writable()
+- os.is_executable()
+- os.is_link()