history.rst: fix wrong subcommand name and unconventional braces

For alteration we usually use "(a | b)", not "{a | b}".

While at it, instead of writing 4/6 subcommands in one line, write them
on separate lines, so it's very obvious that all these are separate
subcommands. We mainly use the (a | b) syntax for long/short options.

doc_src/cmds/history.rst

@@ -11,7 +11,10 @@ Synopsis
 	\	\   [ *SEARCH_STRING...* ]
 | ``history`` **delete** [**--case-sensitive**]
 	\	\   [**--exact** | **--prefix** | **--contains**] *SEARCH_STRING...*
-| ``history`` { **merge** | **save** | **clear** | **clear_session** }
+| ``history`` **merge**
+| ``history`` **save**
+| ``history`` **clear**
+| ``history`` **clear-session**
 
 Description
 -----------