rules.js matching `heading X` instead of `headingX`

[schumannd]

In this line, there is a space that should not be there. I guess the bug was introduced in a refactoring after the package was released, without testing the code again.

I only stumbled upon it in my quest to fork this repository and use the latest simple-markdown version which fixes #92

react-native-simple-markdown/src/rules.js

Line 99 in 277a41b

const stylesToApply = [styles.heading, styles[`heading ${node.level}`]]

I have included a fix to this in my PR #93