Fix a typo ("four" instead of "two")

This is an omission that was supposed to be part of fortran-lang#60, but we missed
it. This PR fixes it.

Author: certik

STYLE_GUIDE.md

@@ -29,7 +29,7 @@ This style guide is a living document and proposed changes may be adopted after
 By setting and following a convention for indentation and whitespace, code reviews and git-diffs can
 focus on the semantics of the proposed changes rather than style and formatting.
 
-* The body of every Fortran construct should be indented by __two (4) spaces__
+* The body of every Fortran construct should be indented by __four (4) spaces__
 * Line length *should be limited to 80 characters* and __must not exceed 132__
 * Please do not use <kbd>Tab</kbd> characters for indentation
 * Please remove trailing white space before committing code