Remove extra space, make compound adjective

Author: ttmarek

react/README.md

@@ -450,7 +450,7 @@
     ```
 
   - Do not use underscore prefix for internal methods of a React component.
-  > Why? Underscore prefixes are sometimes used as a convention in other languages to denote privacy. But, unlike those languages, there is no native support for privacy in JavaScript, everything is public. Regardless of your intentions,  adding underscore prefixes to your properties does not actually make them private, and any property (underscore prefixed or not) should be treated as being public. See issues #1024, and #490 for a more in-depth discussion.
+  > Why? Underscore prefixes are sometimes used as a convention in other languages to denote privacy. But, unlike those languages, there is no native support for privacy in JavaScript, everything is public. Regardless of your intentions, adding underscore prefixes to your properties does not actually make them private, and any property (underscore-prefixed or not) should be treated as being public. See issues #1024, and #490 for a more in-depth discussion.
 
     ```jsx
     // bad