doc: highlighted the func name (#321)

README.md

@@ -3,7 +3,7 @@
 Runtime type checking for React props and similar objects.
 
 You can use prop-types to document the intended types of properties passed to
-components. React (and potentially other libraries—see the checkPropTypes()
+components. React (and potentially other libraries—see the `checkPropTypes()`
 reference below) will check props passed to your components against those
 definitions, and warn in development if they don’t match.