syntax: allow capital letters in Rust struct/trait names

Per https://doc.rust-lang.org/reference.html#fn2, non-ASCII characters are
currently not allowed in stable Rust, so keep it ASCII-only for now.

Signed-off-by: Dirkjan Ochtman <dirkjan@ochtman.nl>

doc/syntax/rust.nanorc

@@ -20,7 +20,7 @@ color red "[a-z_]+!"
 color magenta "[A-Z][A-Z_0-9]+"
 
 # Traits/Enums/Structs/Types/...
-color magenta "[A-Z][a-z0-9]+"
+color magenta "[A-Z][A-Za-z0-9]+"
 
 # Strings
 color green "\".*\""