some mistakes on equations.html

hi @mathexl 

in equations.html after the `Propane, C3H8` equation you have showed `Propane, C3H8` but it has `4 Carbon` and `10 Hidrogen` atom so title is wrong for this example. 

the error is in the line ` 240`
`<h1>Propane, C<sub>3</sub>H<sub>8</sub></h1> `. It must change to `<h1>Butane, C<sub>4</sub>H<sub>10</sub></h1> `
 