Update README.md

Added description for pressure conversion (unit vs. abbreviation isn't that easy to handle for non native english speakers, hope its's now correct).

Author: WRPSoft

README.md

@@ -434,15 +434,15 @@ The following units and their abbreviations can be used (each table can only con
   </tr>
   <tr>
     <td rowspan="14" align="center">Pressure</td>
-    <td>Bar</td>
+    <td>bar</td>
     <td>bar</td>
    </tr>
   <tr>
     <td>PSI</td>
     <td>psi</td>
   </tr>
   <tr>
-    <td>ATM</td>
+    <td>atm</td>
     <td>atm</td>
   </tr>
   <tr>