Correct prefix for "kilo" should be small k, not capital K (#13714)

Ref: https://en.wikipedia.org/wiki/Kilo-

For example in the Fanspeed values 4500 RPM (Rounds Per Minute) should be written as 4.5 kRPM, not 4.5 KRPM.

Regards
GG

LibreNMS/Util/Number.php

@@ -43,7 +43,7 @@ class Number
         }
 
         if ($value >= '0.1') {
-            $sizes = ['', 'K', 'M', 'G', 'T', 'P', 'E', 'Z', 'Y'];
+            $sizes = ['', 'k', 'M', 'G', 'T', 'P', 'E', 'Z', 'Y'];
             $ext = $sizes[0];
             for ($i = 1; (($i < count($sizes)) && ($value >= 1000)); $i++) {
                 $value = $value / 1000;