update grep to be more specific (#6794)

* update grep to be more specific

when grepping for openfiles the netdata example is not exact enough and will return results from too many processes, just adding `.pid` to the grep _should_ pick up the correct process.

* Update high-performance-netdata.md

* Update high-performance-netdata.md

docs/high-performance-netdata.md

@@ -126,7 +126,7 @@ You can check limits with following commands:
 
 ```sh
 cat /proc/$(ps aux | grep "nginx: master process" | grep -v grep | awk '{print $2}')/limits | grep "Max open files"
-cat /proc/$(ps aux | grep "netdata" | head -n1 | grep -v grep | awk '{print $2}')/limits | grep "Max open files"
+cat /proc/$(ps aux | grep "\/[n]etdata " | head -n1 | grep -v grep | awk '{print $2}')/limits | grep "Max open files"
 ```
 
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