README: range-end is not a flag and should not be used

As README declares in the beginning, flags can have dash and it should be
replaced with underscore while using. Also flags or commands are emphasized
in general. The range-end is not a flag or valid doc word and should not be
used as such.

etcdctl/README.md

@@ -83,7 +83,7 @@ Insert '--' for workaround:
 
 ### GET [options] \<key\> [range_end]
 
-GET gets the key or a range of keys [key, range_end) if `range-end` is given.
+GET gets the key or a range of keys [key, range_end) if range_end is given.
 
 RPC: Range
 
@@ -178,7 +178,7 @@ If any key or value contains non-printable characters or control characters, sim
 
 ### DEL [options] \<key\> [range_end]
 
-Removes the specified key or range of keys [key, range_end) if `range-end` is given.
+Removes the specified key or range of keys [key, range_end) if range_end is given.
 
 RPC: DeleteRange
 
@@ -339,7 +339,7 @@ Prints the compacted revision.
 
 ### WATCH [options] [key or prefix] [range_end] [--] [exec-command arg1 arg2 ...]
 
-Watch watches events stream on keys or prefixes, [key or prefix, range_end) if `range-end` is given. The watch command runs until it encounters an error or is terminated by the user.  If range_end is given, it must be lexicographically greater than key or "\x00".
+Watch watches events stream on keys or prefixes, [key or prefix, range_end) if range_end is given. The watch command runs until it encounters an error or is terminated by the user.  If range_end is given, it must be lexicographically greater than key or "\x00".
 
 RPC: Watch