.editorconfig: fix character encoding property specification

The property should be named charset, not encoding, according
to editorconfig documentation.

https://editorconfig.org/#supported-properties

Signed-off-by: Michal Lenc <michallenc@seznam.cz>

.editorconfig

@@ -2,7 +2,7 @@
 root = true
 
 [*]
-encoding = utf-8
+charset = utf-8
 end_of_line = lf
 insert_final_newline = true
 trim_trailing_whitespace = true