Fix colour coding for character literals in F#.

In this code, this match will catch too much and too much would be colored as `String`. Capturing "." or "\\." should be enough for char literals, as they can only be one char long. 
Some text to test against

    let trim (str:String) =
        str.Split( [| '\n' ; '\r' |], StringSplitOptions.RemoveEmptyEntries)
    let unicodechar = '\u1234'
    let utfchar = '\U1A343214'
    let simplechar = 'a'
    let ``shouldnt catch`` = '\uwert' // not legal
    let var' = var + 1 // Shouldn't mess with var'
    let sillychar = '\uabcd' // legal
    let sillychar2 = '\uABCD' // legal

    let chars = [| 's'; 'b'; 'c' |]

This is what it looks like in Xamarin Studio:
![screen shot 2013-07-09 at 11 00 59](https://f.cloud.github.com/assets/445888/767084/3b4196fe-e876-11e2-81f2-417c5e193893.png)

This is in MonoDevelop after applying the patch:

![screen shot 2013-07-19 at 01 20 40](https://f.cloud.github.com/assets/445888/823068/bf9cda80-f000-11e2-9393-9d680f31edcf.png)

cc @mkrueger