Pretty print kilobytes using kB instead of KB (#732)

Apparently postgres requires a lowercase `k`.

I got this error:
```
13:45:13 26 ERROR 2021-06-17 13:45:03.277 GMT [32] LOG: invalid value for parameter "effective_cache_size": "5993 KB"
13:45:13 27 ERROR Failed to install pg_auto_failover in the monitor's Postgres database, see above for details
13:45:13 26 ERROR 2021-06-17 13:45:03.277 GMT [32] HINT: Valid units for this parameter are "kB", "MB", "GB", and "TB".
13:45:13 26 ERROR 2021-06-17 13:45:03.277 UTC [32] FATAL: configuration file "/tmp/monitor/postgresql-auto-failover.conf" contains errors
```

Author: JelteF

src/bin/pg_autoctl/system_utils.c

@@ -114,7 +114,7 @@ pretty_print_bytes(char *buffer, size_t size, uint64_t bytes)
 {
 	const char *suffixes[7] = {
 		"B",                    /* Bytes */
-		"KB",                   /* Kilo */
+		"kB",                   /* Kilo */
 		"MB",                   /* Mega */
 		"GB",                   /* Giga */
 		"TB",                   /* Tera */