fixed missing semicolon

Added a semicolon to fix the following compilation error:

```
➜  mucell git:(master) cargo build
   Compiling mucell v0.1.3 (file:///Users/awri10/src/rust/mucell)
/Users/awri10/src/rust/mucell/src/lib.rs:235:72: 235:73 error: macros that expand to items must either be surrounded with braces or followed by a semicolon
/mucell/src/lib.rs:235 impl_fmt!(Show Octal Binary LowerHex UpperHex Pointer LowerExp UpperExp)
```