Clarify blank line rule in newlines_styleguide.md

To clarify what's meant by "from a logical perspective" here, I
consulted Python's PEP8 style guide, which provides some helpfully
precise language: 

> Extra blank lines may be used (sparingly) to separate groups of 
> related functions. Blank lines may be omitted between a bunch of 
> related one-liners (e.g. a set of dummy implementations).

https://www.python.org/dev/peps/pep-0008/#blank-lines

I adapted this passage to the existing language for the newline rule.

doc/development/newlines_styleguide.md

@@ -2,7 +2,7 @@
 
 This style guide recommends best practices for newlines in Ruby code.
 
-## Rule: separate code with newlines only when it makes sense from logic perspectice
+## Rule: separate code with newlines only to group together related logic
 
 ```ruby
 # bad