small fix to reaction display

This will allow symbolic stoichiometry to render in the reaction,
provided that the value has a sign specified. For example,

```python
import cobra.test
from sympy import Symbol
m = cobra.test.create_test_model()
x = Symbol("x", positive=True)
m.reactions.PGI._metabolites[m.metabolites.f6p_c] = x/2
m.reactions.PGI.reaction
```
will output
```g6p_c <=> x/2 f6p_c```