doc: Fix opindex for -W options

@opindex -Wxxx is wrong; it should be @opindex Wxxx.


	* doc/invoke.texi (Warning Options): Fix some @opindex syntax.

From-SVN: r272672

gcc/ChangeLog

+2019-06-26  Segher Boessenkool  <segher@kernel.crashing.org>
+
+	* doc/invoke.texi (Warning Options): Fix some @opindex syntax.
+
 2019-06-26  Martin Liska  <mliska@suse.cz>
 
 	PR tree-optimization/90973

gcc/doc/invoke.texi

@@ -6146,8 +6146,8 @@ false positives and is deactivated by default.
 
 @item -Wattribute-alias=@var{n}
 @itemx -Wno-attribute-alias
-@opindex -Wattribute-alias
-@opindex -Wno-attribute-alias
+@opindex Wattribute-alias
+@opindex Wno-attribute-alias
 Warn about declarations using the @code{alias} and similar attributes whose
 target is incompatible with the type of the alias.
 @xref{Function Attributes,,Declaring Attributes of Functions}.