[ruby/psych] docs: fix simple typo, expessed -> expressed

There is a small typo in ext/psych/yaml/yaml.h.

Should read `expressed` rather than `expessed`.

ruby/psych@1150d669cb

ext/psych/yaml/yaml.h

@@ -1734,7 +1734,7 @@ typedef struct yaml_emitter_s {
         size_t length;
         /** Does the scalar contain line breaks? */
         int multiline;
-        /** Can the scalar be expessed in the flow plain style? */
+        /** Can the scalar be expressed in the flow plain style? */
         int flow_plain_allowed;
         /** Can the scalar be expressed in the block plain style? */
         int block_plain_allowed;