Fixed minor inconsistency with the html command and added usage example.

If the command now gets issued without parameters the bot will instead print out the command usage *nix style. 
```
gryyy: `html
Bot: gryyy: Usage: `html <elementName>. Example: `html div
gryyy: `html div
Bot: gryyy: The HTML <div> element (or HTML Document Division Element) is the generic container [...]
```
I also corrected a minor inconsistency that you mentioned in IRC where you were using two different values for the length comparison and the substring.