doc/syntax: escape '@' to fix a warning.

Fix the following warning:
doc/syntax.texi:147: warning: Accent command `@"' must not be followed by whitespace

doc/syntax.texi

@@ -144,7 +144,7 @@ The undefined value can be expressed using the "0:0" string.
 @section Color
 
 It can be the name of a color (case insensitive match) or a
-[0x|#]RRGGBB[AA] sequence, possibly followed by "@" and a string
+[0x|#]RRGGBB[AA] sequence, possibly followed by "@@" and a string
 representing the alpha component.
 
 The alpha component may be a string composed by "0x" followed by an