man: ip-netns.8: Clarify synopsis a bit

Use brackets to show that 'ip netns' defaults to action 'list', drop
superfluous curly braces around 'set' action keyword.
Signed-off-by: Phil Sutter <phil@nwl.cc>

man/man8/ip-netns.8

@@ -13,7 +13,7 @@ ip-netns \- process network namespace management
 .BR help " }"
 .sp
 .ti -8
-.BR "ip netns" " { " list " } "
+.BR "ip netns" " [ " list " ]"
 
 .ti -8
 .B ip netns add
@@ -24,7 +24,7 @@ ip-netns \- process network namespace management
 .RI "[ " NETNSNAME " ]"
 
 .ti -8
-.BR "ip netns" " { " set " } "
+.B ip netns set
 .I NETNSNAME NETNSID
 
 .ti -8