docs: fix minor typo on pmunregisterderived.3 man page

Author: natoscott

man/man3/pmunregisterderived.3

@@ -15,7 +15,7 @@
 .\"
 .TH PMUNREGISTERDERIVED 3 "" "Performance Co-Pilot"
 .SH NAME
-\f3pmUnregisterDerived\f1  \- unregister a derived by metric
+\f3pmUnregisterDerived\f1  \- unregister a derived metric
 .SH "C SYNOPSIS"
 .ft 3
 .ad l